The Byzantium was a category four, Galaxy Class, Star Liner.
History
The Eleventh Doctor, River Song and Amy Pond witnessed the Byzantium crash during the 51st century. It carried a Weeping Angel and when the ship crashed, the Angel was let loose. (DW: The Time of Angels) It transpired that the Angel had sabotaged the ship and forced it to crash, so its starving brethren on Alfava Metraxis could feed off the ship's energy: once crashed, the ship began to emit radiation from its drives, harmful to humans and a source of food for the Weeping Angels. However it was later revealed that on its secondary flight deck it had a crack in the universe; the real motive for the Angels to crash the ship on Alfava Metraxis.
The Byzantium was able to travel for 10 years before the need for planetfall, partially due to it having an enormous forest within itself, with modified trees to convert starlight into oxygen for the crew to breathe. (DW: Flesh and Stone)
River Song mentions that to Amy Pond that she just returned from the Byzantium when telling her that the Doctor didn't really die and that she is married to him. (DW: The Wedding of River Song)
When River Song met the Tenth Doctor just before her death, she mentioned the crash of the Byzantium. (DW: Silence in the Library)