Those that want to use the GUI should install mkvtoolnix-gui.
Matroska is a new multimedia container format, based on EBML (Extensible Binary Meta Language), which is a kind of binary XML.
The mmg tool is a graphical user interface for the mkvmerge program, which allow one to create Matroska files from other formats.
This package also contains a graphical version of the mkvinfo tool, which allow one to get information about a Matroska file.
Please let us know how it's working so we can move it to main if it's qualified.