The Nebulon-B Frigate is a warship from the Star Wars Universe that is often referred to as either a Rebel Cruiser or a Rebel Medical Frigate.
The ship was first seen in the ending of the movie, The Empire Strikes Back. It is where Luke Skywalker is seen recovering after having his hand replaced by a cybernetic replacement after losing his hand in a fight with Darth Vader.
In universe, the vessel is designed to serve as an escort vessel to defend cargo ship from attack by Rebel starfighter attacks. It was deployed by the Empire but due to a large number of the ships being captured or defecting to Rebel forces they serve as a major part of the Rebellion's navel forces.
The vessel is a multi-role ship armed with both Turbolasers(12) and light Anti-Starfighter Heavy Blasters(12). The military configuration has a hanger for two squadrons of starfighters, this section is replaced by medical facilities in the Rebellion's Medical Frigate, Redemption. The Hanger Deck is not large enough for landing larger vessels including light freighters so in the neck of the ship is located a docking port to allow those vessels to interface with the ship when necessary.
In the Tie Fighter game and sequels varied versions of the vessels are seen including the modified frigate that has two instead of one forward booms. Since the boom contains most of the weapons on the ship it is presumed that it has almost twice the normal armaments of the original ship.