What is .mkv files ?
MKV (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 graphical user interface for the mkvmerge program, which allows to create Matroska files from other formats.
to install from terminal :
sudo apt-get install mkvtoolnix
OR Download and install manualy from here :
to start : Applications - Audio & Video - MKV files creator