What is a .mkv file?
"What is .mkv file? I thought I downloaded a movie, but when I got the file it is a .mkv. I don't have anything that will play the file."
MKV is the file extension used for Matroska container format video files. MKV is like AVI or MP4 which can contain several types of audio or video inside a common container. The required codecs for video and audio may be different from file to file, so it's best to simply install a video player with wide support for almost all formats instead of trying to make your current video player work. Here is what I recommend.
The easiest solution for playing MKV files is VLC Player. In most cases VLC Player will playback every MKV file you throw at it. The quick and easy solution is to download and install VLC Player, then launch VLC Player and open the .mkv file you want to play.
There are a few cases where the file won't play in VLC Player. Most of the instances where VLC Player won't play MKV files are because the video codec used is RMVB. If your MKV file won't play in VLC Player, the next best thing to try is RealPlayer SP, which plays back all files based on the codecs created by Real Networks.