Although originally intended as a vessel of the Imperial Navy, enough escort frigates are in use by the Alliance that they are informally known as Rebel cruisers.
Manufactured by Kuat Drive Yards -- makers of the mighty Star Destroyers -- the Nebulon-B escort frigate was designed as an affordable capital ship meant to handle the threat of Alliance starfighters. Its launch bays can hold two squadrons of starfighters and the ship carries specialized weaponry designed to hit small targets.
Through defection of Imperials and a number of notable victories, the Alliance has been steadily acquiring frigates throughout the course of the Galactic Civil War. Many are refitted for other duties -- such as Rebel medical frigates. In these vessels, Alliance techs have replaced the launch bays with hospital suites. After sustaining injuries at Cloud City, Luke Skywalker was treated aboard a medical frigate.