Описание тега mkv
This tag is for questions about playback of, creation of, and programmatic interaction with, Matroska multimedia Container (*.mkv,*.mka) files.
MKV refers to Matroska Multimedia Container format, an open standard free container format, that can hold an unlimited number of video, audio, picture or subtitle tracks in one file. It is intended to serve as a universal format for storing common multimedia content.
For more details see http://en.wikipedia.org/wiki/Matroska