Rebel transports are no-frills vessels that are little more than flying cargo carriages equipped with hyperdrives. The open underbelly of the Rebel medium transport has room for scores of cargo containers. The vessels have minimal armament, and are relatively slow. They travel in convoys for protection, often escorted by X-wing starfighters and larger vessels.
During the evacuation of Hoth, these transports were pivotal in moving command personnel and important supplies away from the battle zone. Since resources were spread so thin -- as is typical for the Alliance -- these transports had a bare minimal escort of two X-wing fighters each. The Alliance's giant ion cannon provided the added protection of cover fire during the evacuation.
These ships are Gallofree Yards medium transports that the Alliance purchased at greatly reduced prices. Although affordable, these transports do require a fair amount of maintenance.
With the addition of a tan-colored transport docked in Mos Eisley in the Special Edition version of A New Hope, these vessels now appear in each film of the classic trilogy.