A new video and audio container format similiar to AVI but with several new features like support for OGG audio, Variable Framerate Video.
Matroska File Formats:
.mkv : Generally video files, as well those containing audio ( movies ) or video only
.mka : audio only files, can contain any supported audio compresion format, such as MP2, MP3, Vorbis, AAC, AC3, DTS, PCM and soon MPC ( musepack )
.mks : a so called 'elementary' matroska stream containing any subtitles stream
For more info, see http://www.matroska.org