MKV's name comes from matryoshka, the Russian nesting dolls. MKV is a video format that works as a container, not as a codec. MKV files can contain a wide range of video, audio, picture or subtitle items within a single item.
MKV files may also feature integrated decoders, but they usually require a specific codec or player in order to be played, so users will usually convert them to other formats that may be played and edited without any additional requirements.
MKV converting apps will also allow their users to encode their movies to common video formats such as AVI, MPEG or WMV, files that may be later played on most players and portable devices.