VLC is an open source, cross platform media player application for video and audio. This tag should be used for questions that are specifically about the VLC application. In addition to this tag an Operating System specific tag should also be used since VLC is a cross platform application. The more generic media-player tag may also be helpful.
The VideoLAN official site describes VLC as "a free and open source cross-platform multimedia player and framework that plays most multimedia files as well as DVDs, Audio CDs, VCDs, and various streaming protocols."
As of VLC Rel 2.2 the support for Bluray disk playback requires additional Bluray decoding libraries which must be obtained elsewhere.
See as well the Wikipedia topic VLC media player.