The M-ships are the class of ships used by the Ravagers.
Peter Quill used a M-Ship called the Milano to start his career as a Ravager.
Battle of Xandar
During the Battle of Xandar, Yondu Udonta planned to use his M-ship to enter through a hole to the Dark Aster, together with the Milano. On approach to the Dark Aster, the ship was damaged by a Necrocraft, losing altitude and crashing on Xandar.
Meanwhile, Kraglin Obfonteri, Horuz and other Ravager Pilots used their M-ships to protect the city from dive-bombing Necrocrafts. Horuz's ship was destroyed when it was struck by one of the Necrocraft, killing both Horuz and his co-pilot.