Usually, I find there's nothing that VLCPlayer can't handle. Whenever I download or find an unusual video file type, I can always rely on VLCPlayer to open it. However, that's not the case with MKV files.

If you've never heard of it then you will soon as MKV is a format rapidly growing in popularity due to its high compression rates and excellent reproduction quality. MKV is just one format that's part of the Matroska Multimedia Container standard is an open standard free container format. According to Wikipedia such formats can:

Hold an unlimited number of video, audio, picture or subtitle tracks inside a single file. It is intended to serve as a universal format for storing common multimedia content, like movies or TV shows. Matroska is similar in conception 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.

While open source players such as VLC should have no problem playing such files, according to After Dawn:

While VLC will work for most MKV files you will find, the package does not include any proprietary codecs. RealVideo and ON2 VP6 are examples of codecs not packed with VLC for this reason. Therefore with VLC, if you get a Matroska file that contains RealVideo, you are pretty much in the dark and will have to look for another playback option.

The simple answer I found was to download the CCCP codec pack which provides support for a whole range of media formats including all possible Matroska formats. With this installed, VLCPlayer should be able to handle it but if it doesn't, Windows Media Player will.