Nebulon Class B Frigate

Star Wars Lore
From the OS Databank.

Movies
Unlike the Empire, which had a galaxy of resources to draw upon, the Rebellion was hard-pressed to equip its soldiers and fleets. The Alliance operated what few cruisers they had non-stop, often modifying them to fit mission profiles beyond their original specifications.

Expanded Universe
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. It came as response to repeated Alliance raids on Imperial convoys. Accompanied by escort frigates, the rich prizes of Imperial shipping suddenly became risky ventures to Alliance tacticians. The escort frigate's launch bays can hold two squadrons of starfighters and the ship carries specialized weaponry designed to hit small targets. In response to this new threat, the Rebels developed the heavily-armed B-wing starfighter.

Through defection of Imperials and a number of notable victories, the Alliance steadily acquired frigates throughout the course of the Galactic Civil War. Enough escort frigates were in use by the Alliance that they informally were known as Rebel cruisers. Many were 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.