MKV Files
Explained in Plain English

MKV is video file format that began surging in popularity in 2007. (See Wikipedia for a complete technical discussion.)

On Windows, you can play MKV files with many different video players, including the latest version of Windows Media Player, and the less-annoying Media Player Classic that comes with the freeware K-Lite Code Pack.

On the Mac, you can play MKV files with the versatile, and free, VLC.