Wikipedia
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 in one 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 Advanced Systems Format (ASF), but is entirely open in specification, with implementations consisting mostly of open source software. Matroska file extensions are .MKV for video (with subtitles and audio), .MK3D for stereoscopic video, .MKA for audio-only files, and .MKS for subtitles only.
"Matroska" is derived from Matryoshka , which refers to the hollow, wooden, "Russian doll", or Matryoshka doll, that opens to expose another doll that in turn opens to expose another doll, and so on. The logo uses "Matroška", with the caron over the "s", as the letter š represents the "sh" sound (as in "Matryoshka") in the Slavic languages in which it is used.