Ship Classes

Over centuries, the same names for ship classes have changed meaning. A frigate was originally a full-rigged ship built for scouting and commerce raiding; in the 21st century it refers to an escort for larger warships. Star Fleet has its own meaning of these terms, but as is it's wont Star Fleet uses historical terms where possible.

Starships

Cruisers and Frigates are most of the starship-class vessels. A heavy starship is capable of sustained Warp Factor 8 and has a reactor output greater than 2.5 terawatts. A light unit is capable of sustaining warp 7, and has a 2-2.5 terawatt reactor.

Cruisers are definitionally multi-role ships. There are various sub-types (heavy & light cruisers, survey cruiser, battle cruiser, etc.). Terran cruisers must be able to perform all of these missions: Combat, both independently and as part of a squadron; long range scouting; and planetary assault. Technologically, that means they have to have considerable endurance (at least one year without a port call), long range sensors (and the accompanying sensor dish), heavy weapons (torpedo tubes), and an embarked unit of commandos, with equipment. Other powers’ starships are classified as cruisers if they are intended as general-purpose units, rather than specialized. 

Frigates are more specialized and collectively are all the starships that aren’t capable of all the cruiser missions. Sub-types include torpedo frigates, troop frigates (for raids & special operations), fast (packet) frigates, scout frigates, etc. There are even a few medical frigates.

Spaceships

Destroyers are designed for escort duty and for missions that require firepower but not the full capabilities of a larger ship, such as planetary bombardment. In practice there’s often little difference between destroyer and cruiser armament; the difference is in available power. Destroyers have reactor outputs in the 1-1.5 terawatt range and are capable of warp 6. They are the smallest units to conduct interstellar voyages routinely.

Corvettes are long-endurance craft (crew intended to live aboard). Their main purpose is system defense and policing, but they are capable of short interstellar voyages. They have a reactor output of 500-750 megawatts and cruise at warp 5.

Gunboats are short-endurance craft, usually only capable of low warp speed and missions of a few days. They are intended for local space control; redeploying to another solar system would take months, or a specialized tender. They reach warp 4 on a 500 megawatt reactor.