this is my take on ship classification. It differs quite a bit I think from others. This classification system is also colored a bit by my hard-SF bent. Aurora is still awesome though.
Fighter- small craft, very fast and hard to hit. Like a recoverable missile bus (missile that shoots missiles), that is crewed. A "gunboat", "gunship", "assault boat/boarding craft", "fighter/bomber", and "bomber" would all be classed as fighters. So would "interceptors".
Destroyers- small vessels with frigate-level firepower, but very little long-term endurance. Typical mission lengths are up to 6 months at most, more typically 2-8 weeks. In other words- good for home patrol and defense, bad for offensive operations. They have some multirole usage, mainly as combatants, point defense, and sensor ships.
Troop Transport- pretty much, TTs are just lightly armed frigates. They swap weapons space for troop carriage capability. They can either carry 500-2000 power-armor troopers, or a 150-500 man armored assault force. Armored assault forces are primarily used for fire support in planetary operations (like artillery). TTs are also useful as logistics vessels, carrying spare parts, additional ordnance, and extra food/water/air/fuel, due to their capacious internal bays.
Frigates- sizeable vessels, these are essentially small battleships. They are smaller than battleships, with a proportionately smaller firepower and survivability "rating". They have excellent (8-24 months) endurance, and have high delta-vs (change in velocity, meaning can go a lot of places). Frigates can perform a variety of roles, including: fleet combatant, fighter carriage, point defense, sensor net operations, C4ISR (Command, Control, Communications, Computers, Intelligence, Reconnaissance, Surveillance), and planetary raids.
Pocket Battleship- approximately midway between frigates and full-sized battleships, they have battleship-level firepower and usually survivability, but are smaller, require less crew, and tend to have greater endurances (12-36 months, though 48-96 months is possible) than frigates and even some larger warships. The reasoning behind a pocket battleship is- fewer crew, fewer pieces, smaller ship (for material frugality).
Battleship- this is a beast of warfare. Note that, if you have the resources to build a fleet of battleships, the only reason not to is so you can have more frigates/pocket battleships in more places at once. Endurance is very long (48 months is normal).
Supercarrier- these are inferior to battleships in many respects, but do have the advantage of- huge troop bays, huge hangar bays, and massive ordnance and supply carriage. Basically, supercarriers are really big troop tranports, but with frigate-sized guns (and lots of them).
