“MKV” files – or Matroska files- files can be played with QuickTime using a free component called Perian.
Perian is called the Swiss- Army knife of QuickTime® components because it enables QuickTime to work with many different video and audio formats. It’s a free download at the link above.
Perian will also handle file formats like AVI, DIVX, FLV, MKV, GVI, VP6, and VFW and many video, audio and subtitle formats, too, so it’s a great component to have.
According to Wikipedia, the Matroska Multimedia Container is an open standard free container format, a file format that can hold an unlimited number of video, audio, picture or subtitle tracks inside a single file. It is intended to serve as a universal format for storing common multimedia content, like movies or TV shows.