Frigate is a type of Starfleet Ship.
The term Frigate is from Earth naval history.6th to 4th rate. The role they filled was that of independent patrol, or fleet picket work. Later in time the former task fell to cruisers and the latter to the destroyer. The term has come back into fashion in modern navies for ships smaller than destroyers. Destroyers now being as large as cruisers used to be and filling both their picket role and that of independent patrol.
23rd centuryEditIn the 23rd century, A Frigate was the smallest practical stand alone starship. These were war of attrition units, intended to stretch available manpower and resources as far as possible. These were used in groups to escort larger ships and to conduct anti-piracy patrols. Frigates were modified into special purpose units, but not very often since their small hulls didn't take well to such modifications. Service on Starfleet Frigates was often described as "nasty, brutish and short", since the ships lacked many amenities of larger starships.
Some ships retained the designation, even though they actually had the size and firepower of cruisers.
The 24th century Starfleet was reorganized away from the "War with the Klingons" Model that dominated the 23rd century.
As such the terms Destroyer , Strike Cruiser , Battlecruiser , Dreadnought and Battleship were retired, being viewed as too militaristic and liable to present problems to new contacts when run through a universal translator.
The term Frigate was redefined to mean any stand alone starship with a primarily military mission, but capable of Multi-role missions, replacing the term "Battlecruiser, strike cruiser, dreadnought." etc.
Ironically most of the 23rd century Frigates would actually be classified as Escorts under the new naming scheme.