Home Theatre PC. Usually a small, low power computer that connects to a home theatre system (HDTV + speakers/amp). It gives the user the ability to play DVD/Blu-ray films directly but also video files saved on the hard disk and in many cases streamed video files from the internet (such as BBC's Iplayer). Playing audio CDs and stored audio files is also well within their capability. Also, depending on the specification they can play games on the massive TV - which is always fun.
All-in-all pretty cool things to have if you have a home theatre setup.