In short, matroska is a new Audio/Video file format. It is an advanced
and full featured format.

Advanced because it is based on EBML, a kind of XML equivalent, that
allow infinite extensibility of the format. And full featured because
it includes precise seeking, any audio/video/subtitle codec support
including VCM/ACM/DirectShow compatibility, timecode based format,
complex frame dependencies, chaptering, internationalisation, error
protection, tagging, file attachement, control track (to be defined),
menu (to be defined), etc. All these features are not yet implemented
but already defined in the format.