[ excerpt from developer's site ]

matroska is a project aiming to become the standard of Multimedia
Container Formats one day. It was derived from a similar project
called MCF, but differentiates from that significantly because it
is based on EBML (Extensible Binary Meta Language ), a kind of
binary version of XML. This way the development team gains significant
advantages in terms of future format extensability, without breaking
file support in old parsers.

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.