What is MKV file?
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 first became popular in the anime fan-subbing community (where users create subtitles) since it supports multiple audio and subtitle tracks in the same file. It is intended to serve as a universal format for storing common multimedia content, like movies or TV shows.
Matroska is similar in concept to other containers like AVI, MP4 or ASF, but is entirely open in specification, with implementations consisting mostly of open source software. Matroska file types are .MKV for video (with subtitles and audio), .MKA for audio-only files and .MKS for subtitles only.