A .mkv file is a Matroska video/audio multimedia file. It is a container like MOV and AVI, providing support for a huge number of different video, audio and subtitles compression formats. It is not really intended for media content distribution on a large scale, but is achieving it anyway.
If you want to learn how to play MKV files on your computer, then read our comprehensive "How to Play MKV files" guide.
You can also learn how to play MKV on an Xbox 360 in our "How to play MKV content on Xbox 360" guide.
MKV's popularity is partly due to its incredible support for several subtitle formats including SRT, SSA and USF text subtitles, or VobSub streams coming directly from the DVD. The team is also currently working on a DVD-like menu structure for the format.
There are several players and other tools for Matroska files available to download from AfterDawn.
Related software tools
Arpa Kolla player (Freeware)
Arpa Kolla is a standalone freeware DVD/svcd/vcd/mpg/avi/ogm/mkv/xcd player with srt support.
Avi2DVD is a GUI in order to convert with just few clicks an AVI/OGM/MKV/DVD to DVD/SVCD/VCD.
Matroska Splitter (Open source)
Matroska Splitter is a DirectShow splitter for Matroska (.mkv) video files. You need this in order to open .mkv files.