Can you download a DivX codec for Apple TV that will allow you to play Matroska file extensions? PS3 won't play them either (nothing Sony makes will), but Xbox 360 does.
Matroska container, same as any other container files (MOV, AVI, etc.). It's a container that holds both an audio and video file. MKV is popular because it can also hold subtitle files and can be compressed into a much smaller file size than AVI (making for a quicker download).