What is .mkv file and how to open it?

An MKV file is a video container format similar to the .AVI, .ASF, and .MOV formats. It supports several types of audio and video codecs and may include .SRT, .SSA, or .USF (Universal Subtitle Format) subtitles. MKV files also support VobSub subtitles from DVDs.

How to open .mkv file?