The Byzantium was a spaceship or some other kind of flying object which the Doctor witnessed crashing together with River Song. (DW: Silence in the Library) Apparently a Weeping Angel was being held there during the time of DW: the Time of Angels/Flesh and Stone and the Doctor and River Song fought it, which may have had something to do with the Byzantium's eventual crash.
Behind the Scenes
The crash will apparently be depicted in series 5, as wreckage labeled Byzantium was seen on set during the filming of The Time of Angels/Flesh and Stone.Set pic This seems to confirm that the crash had something to do with the Weeping Angel being held there.
