For something I am writing Starfleet initially uses the command ship, cruiser, frigate and scout to categorize its vessels. Command ships being the biggest and most powerful ships in use in the period, though few were built. Cruisers were tasked with everything from exploration, combat, diplomacy, disaster relief, border patrol and showing the flag operations. Frigates were smaller vessels, often equipped only with beam weaponry to save space and cost, that did all the jobs cruisers were to big for and some of the same ones. Border patrol, anti-piracy patrols, convoy escort, scouting, exploration, and more were all tasked to frigates, which also escorted cruisers and command cruisers in large fleets. Scouts were the smallest ships in the fleet and often carried little, if any armament. They were given the exclusive job of exploration, surveying and long term scientific experiments. Following a war with the Klingons a new fleet was developed with new and different categories of ships. Among them... Battlecruiser Replacing the command cruiser these ships possessed the speed of a cruiser, but staying power of a much larger vessel. As a part of a fleet or battlegroup these ships pummeled enemy vessels at close range, while tanking return fire thanks to thick armor and heavy shielding. Few such ships were built due to their cost, but those ships that were built were often deployed strategically to chokepoints, along probable invasion corridors, along the border, and even for high level diplomacy. Heavy Cruiser A branch off from the cruiser these ships fulfilled a role somewhere between a battlecruiser and light cruiser. Having heavy weapons but none of the thick armour of the battlecruiser. In a battle the ships took up position along the flanks of battlecruisers. Outside of the battlefleet the ships perform many of the duties of a traditional cruiser, but make more of a statement when deployed. Light Cruiser Smaller, cheaper and less well armed than a heavy cruiser these ships are built for economical operations. Being not as fast as a heavy cruiser, but having much better power consumption and fuel economy than the larger ships. They are used for many of the same roles as the heavy cruiser, but do so much cheaper. Older heavy cruisers are often reclassified as light cruisers later in their lifecycles. Light cruisers often also lack the ability to take part in a fleet level engagement in anything other than a support or screening role. Destroyer Fast, well armed, lightly crewed and cheap to construct and operate these ships perform border patrol missions while away from a battlefleet or larger formation. They lack any strong shields and have engines large enough only for brief periods of sustained high speed. In terms of weaponry these ships have a heavy focus on torpedoes, allowing them to punch far above their own size class. Frigate Little changed from before the war these ships perform many of the same missions as the they always have. Although now their role in the battlefleet has been replaced by the destroyer. These ships remain very common in service and are built in very large numbers. Meant as economical ships they are far from fast, but practically sip fuel allowing them to have a very long range. Scout Radically changed from before the war this type of vessel now fulfilles the role of an internal long term scientific vessel optimised for easily changing its scientific equipment and hosting a large number of scientists aboard for extended periods. The ships can be easily configured for a variety of missions from planetary study, stellar research and others. Note that this classification system is really only for Starfleet in the TOS era. Things would likely change significantly by the next generation era. This is also from something a little different from mainstream trek, I am working on it now and it should begin to be posted by sometime early next year.