This was submitted by IlJackson, but since we don't have a place for it, I decided to post it here.
I.L. Jackson's Guide to Ship Classes
As anyone who has glanced at my gallery knows, I am a star ship fan(atic). I love designing my own starships and I know others do too. But, I've noticed that a lot of artists do not have a good idea of ship classes, what they do (traditionally) and their uses, particularly in a fleet. So, here is a guide to ship classes as they have been used traditionally and how they would likely be used in a sci-fi setting.
This is the one a LOT of people get wrong. A Capital ship is an anchor ship for a fleet, meaning that fleets are built around the capabilities of the capital ships. Just about every other ship in the fleet has some job relating to the capital ships, whether it be defending or tending or supplying that ship.
Capital ships tend to be the largest warships in the fleet. Capital ships also virtually NEVER operate alone. A great example is (original universe and original series) U.S.S. Enterprise from Star Trek. It was a heavy cruiser. That means it was not a capital ship. The only capital ships I remember from Star Trek (original series) was the Federation Class from some of the novels. It was a dreadnaught. That's a capital ship. The FASA games and the Starfleet Command games had the Excelsior listed at a battleship, and they had a Space Control Ship (a carrier) those were battleships. The Enterprise-E was also a capital ship.
There are a number of types of capital ships.
Battleships: Battleships are generally the largest ship-to-ship direct fire capital class vessels. They are large, gun-laden ships whose job is to put devastating fire onto a target. They (typically) do not carry more than a handful of fighters, if any. They also do not usually operate alone or away from base for a long period of time unless being used for gunboat diplomacy (scaring the crap out of your enemies and potential enemies by showing off how devastating your firepower is). Battleships are often command ships that are in charge of an entire fleet. The admiral of the fleet often makes this ship his or her (or it's, since we are talking sci fi) flag ship. In generally, unless there's a war on, the battleship does not take to the field. It does not go on non-combat missions (the exception being gunboat diplomacy). This is why the U.S.'s battleships got tarred at Pearl Harbor. They were kept in port until needed. Also, battleships tend to actually be some of the fastest ships in the fleet due to the massive amount of power available. Agility...that's something else. Battleships engage other battleships, destroy carriers, and bombard land fortifications or provide ground force fire support. In a sci-fi setting, this means that they would hit other battleships and carriers, blast space stations, and nuke (or whatever your favorite weapon of mass destruction is) ground-based targets from orbit. If the movie star ship troopers had been realistic, there would have been battleships blasting the bug planets from high orbit for hours (or days or weeks) before ground troops moved in).
Carriers: Carriers carry a large number of fighters and other small vessels into battle. Their job is to hang back from beyond combat range and send out fighters to destroy enemy targets. While interceptors are awesomely cool and flashy, the carriers most potent weapon will be strike fighters or bombers. (we'll get to those much later). Carriers are generally POORLY armed. No one wants their commander to take their carrier into the enemy's face. Carriers are laden with fuel and ordinance and lots of other stuff that goes boom easily. They are also not as well armored as other ships. They have good armor, but anything that takes away from their ability to carry fighters or fuel or weapons is a bad thing. Carriers are also typically fast ships (used to runaway or to get into position in a timely manner). If you are doing a heavy space-fighter universe, carriers would be all over the place. Carriers don't just carry fighters. They carry rescue ships, scout ships, pinnaces, shuttles, etc. Carriers are also often flagships, chosen even over battleships. You need great communications to coordinate several fighter and bomber squadrons. That same technology helps an admiral get a feel for the battlespace and communicate with all the ships. And the carrier stays the hell out of the way of actual combat, and Admirals tend to like that. Missile hits tend to make you spill your tea.
Dreadnoughts: Dreadnoughts are battleships. It's just generally a different name for one. If a fleet is fielding both dreadnoughts and battleships, the dreadnought is probably a battleship on steroids. They are extremely gun-heavy and do little else but shoot big guns. They are also very fast.
Battlecruisers: Battlecruisers are likely capital ships for fleets that aren't very big or advanced. Battlecruisers are smaller than battleships and are really more designed to protect battleships and carriers. However, if you're a spacefaring species on a budget, you can push your cruiser designs up to battlecruiser level. A battlecruiser is probably the largest ship designed for independent operations. If you want a big ship that can fight and cruise around the universe by itself, exploring planets and generally getting into mischief, a battlecruiser will do the job, especially if you are the more aggressive, shoot-first-shoot-again-then-ask-survivors-questions type of navy.
*Special Note: Battlecarriers: These don't exist in the real world, but they exist in science fiction. The perfect example being the Battlestar Galactica. These are carrier/battleship hybrids that can carry lots and lots of fighters, and go toe-to-toe with a battleship. These would probably be the biggest ships fielded by a navy (the empire's Executor comes to mind). They would also only be possible in an economy of scale that has a LOT of resources. They would also go into battle with a huge bullseye on their bows. If you could take out someone's battlecarrier, their fleet just lost probably about two-thirds of its punch. If they are sending multiple battlecarriers at you in one fleet, and you don't have some of your own, pack up your little civilization into some fast ships and head the other way. Take lots of pictures of your home world, because you aren't seeing it ever again.
Sub-capital ships (or escort ships)
These ships usually have three roles: First they protect capital ships from other vessels that are designed to kill capital ships (like fighters). Second, they operate as commerce raiders, destroying an enemy's supply lines and resources. Third, they hunt for enemy ships acting as commerce raiders either on their own or in small task forces, or while escorting valuable supply ships. They make sure your big ships don't get sunk, the enemy can't eat, and you can.
Cruisers: These tend to be the largest sub-capital class vessels. In your more peaceful or resource limited fleet, they are the best independent operators. They have long range, good speed, are well-rounded, and carry enough firepower to deter most foes who aren't throwing entire fleets at them. This is why the Enterprise in the original series was a cruiser. If you're going to go seeking out new life and all that jazz, a cruiser is your ship. Unless you live in a really bad neighborhood of the galaxy, in which case see the battlecruisers above. In fleet action, cruisers hang tight to the capital ships and are often bigtime anti-starfighter or defensive weapon platforms. A squadron of bombers coming at your carrier? The cruiser tries to mow them down. A destroyer trying to line up a torpedo run on your battleship? The cruiser tries to put a stop to that. They also would be the command ship for small task forces of other sub-capital ships, leading the charge to blast enemy supply ships. Cruisers would probably come in a lot of varieties. Guided missile cruisers, exploration or scout cruisers, anti-starfighter cruisers, you name it. Variety, variety, variety. There can be light cruisers that are mainly support vessels, heavy combat cruisers, science cruisers, etc. (that ship that delivered the Jedi to the Trade Federation negotiations in Phantom Menace? NOT a cruiser!)
Frigates: These are basically small cruisers, and in fact, they can be so similar that some people have a problem telling the difference. The U.S. Navy had a problem with this a couple decades back when they couldn't decide what was a frigate and what was a cruiser. Frigates are probably going to be smaller and weaker than cruisers (and cheaper), with a medium amount of guns and decent speed. The SSV Normandy, from the space RPG series Mass Effect, is a good example of a frigate in a sci-fi setting. They also have a good amount of variety and purpose-specific designs (the Normandy was a special ops frigate, designed to penetrate enemy lines and deliver special operations teams to conduct crucial operations...if the U.S. had the equivalent today, it's job would be getting Navy Seals close enough to enemy territory to deploy and retrieve them after their mission was done). There would also be scout frigates, frigates designed specifically for patrol duty, etc. They would likely have shorter range than cruisers, so wouldn't be exploring the farthest star.
Destroyers: This is one that garners a lot of confusion. Originally, destroyers were meant to hunt down torpedo boats (little boats with one or two torpedos that would sneak in close and sink capital ships. Nothing like losing your battleship to basically a big speedboat to make you rethink your strategy). Later, they hunted down submarines. Today, they're pretty indistinguishable from frigates. In a hard science sci-fi setting, there is no stealth in space, so destroyers would basically be patrol vessels or anti-starfighter platforms that hug close to capital ships and even cruisers, defending them. In a sci-fi setting with less strict adherences to modern science, or in a very advanced setting where someone has figured out a tech way around the limitations of space stealth as we know it, they would hunt stealth-capable ships down. If the Federation in Star Trek had a destroyer, it would have some advanced sensor system that could spot a cloaked vessel or deploy some kind of weapon that would distort cloaking fields (a tachyon burst torpedo or something...because star trek uses tachyons the way Superman uses "spinning real fast"...they do everything you need when nothing else makes sense). Destroyers would have fairly small and light guns and missiles, one or two few knockout punch weapons with limited payloads, light armor and really, really, really good sensors. (A Star Destroyer from Star Wars? NOT a destroyer)
Escort Carriers (also known as Jeep Carriers): These often get ignored in science fiction, which I think is a shame. They are small carriers that would only have a couple squadrons of fighters, or bombers. You ever wonder where carriers in movies get more fighters after you just saw half their squadrons blown out of the air? That's because there are probably 5-6 escort carriers or jeep carriers in the fleet carrying replacements. They also deliver fighters to bases and act just like traditional big carriers (known as fleet carriers) when there isn't need for a big expensive carrier to do the job. Need fighters to destroy a listening post in an asteroid field or something? Send an escort carrier and a couple destroyers to guard it. Escort carriers can be surprisingly effective in large numbers and if the captains know what they are doing. The last major surface battle in WWII and in human history to date was the battle of Leyte Gulf. The Japanese main surface fleet that included several battleships (including the Yamato...yes that Yamato) and heavy cruisers stumbled upon a little task force known as Taffy-3, which was a few escort carriers, destroyers and destroyer escorts. The Yamato weighed as much as every ship in Taffy-3 combined, and not one gun in the entire Taffy-3 task force could penetrate Yamato's armor. Taffy-3 whipped the Japanese fleet's ass. They deployed every fighter and sent the destroyers charging in, laying down thick smoke. The Japanese admiral thought he was fighting the main U.S. fleet. By the end of the battle, the Yamato turned tail and ran all the way back to Japan. That was its last battle. It was later sunk by fighters. All because of escort carriers and destroyers that were so lightweight they were called "tin cans." Respect the escort carriers.
Destroyer Escorts: These are very small, very lightly armed and armored patrol ships. They escort supply ships, patrol ports and essentially try to be a nuisance to submarines and enemy sub-capital ships. They can do virtually nothing to a capital ship or even a heavy cruiser unless they get very lucky. But if you're trying to get supplies from one place to another and aren't worried about a serious attack, you send some destroyer escorts just to be sure. Destroyer escorts in a sci-fi setting might be fairly indistinguishable from corvettes.
Corvettes: High-speed patrol ships with a couple guns and very short range. They may not even have FTL in some sci-fi settings and may be restricted to one star system, or they may be the smallest ships in the fleet capable of FTL travel. They are lone patrollers and hunter/killers, though they do more hunting than killing. They will often be on picket duty, and their job won't be so much to destroy an enemy ship but to start shouting "HEY! BAD GUY OVER HERE! RIGHT HERE! SOMEONE START SHOOTING THIS GUY OVER HERE PLEASE!" They would also chase down smugglers and pirates (along with destroyers and destroyer escorts). These are the guys that would have actually been sent after ships like the Millenium Falcon and Serenity when they became a problem. Also, pirates, merc and the like could easily afford corvettes and might be able to own one without breaking the law. There are lots of names you can give these types of vessels. Pinnaces, patrol ships, lighters, etc.
Fighters and bombers
Short-range ships of no more than a dozen crew who are launched from space stations, the ground or carriers, go out, do their mission and then go home and land for repair and refueling.
Bombers: Where's the respect for the bomber in sci-fi? It gets ignored a LOT. Carriers main striking power are their bombers. The smallest of these are called Strike Fighters, which can dog fight, and intercept enemy fighters, but whose main job is to destroy enemy assets. They carry big missiles or big guns that can make an enemy carrier commander break out in sweats, especially when there are a few dozen or hundred coming his way. Like big hornets or piranha, they swarm bigger prey and give it a death by a thousand cuts (or by one or two well-placed missiles). Larger bombers are more dedicated to just destroying large enemy ships, space stations and ground-based assets and are more vulnerable to enemy fighters, but they can take a pounding, have larger crews, longer ranges and bigger bombs. In a sci-fi setting, a strategic bomber may have FTL capabilities and operate independently of a carrier, launching from a planet en masse and warping (or hyperdriving or wormholing, whatever) behind enemy lines to bomb an important target. Good use of bombers can win a war without your fleet ever meeting their fleet if you can take out fuel depots, ammunition factories, and if you just want to be a jerk about it, a few dozen might be able to drop nukes all over their favorite headquarters and massive civilian populations, making their people ask "why are we doing this whole war thing again? Seeing my kid disintegrated has made our goals a little fuzzy, o Great Space Emperor safe in your bunker...).
Interceptors: These are the fighters we usually like seeing in action. Fast, nimble, and piloted by guys with perfect hair and girls who smoke cigars and punch commanding officers in the face with no real repercussions. Their whole purpose is to shoot down other fighters and bombers. They "intercept" incoming bad guys who want to blow up all their nice stuff. They need speed to get to the enemy before the enemy can get to them, and because in fighter combat, speed is king. They have just enough fire power to drop a bomber or other fighter, but rarely more than that.
Those are the basic ship classes as we know them today and how they probably should be used in science fiction. You can play with the roles anyway you want, obviously, but if you know what their traditional roles are then what you come up with can make more sense.
But at least you won't be calling frigates capital ships or have your destroyers carrying three hundred fighters and being the largest ship in the fleet.