A set-top box (STB) or set-top unit (STU) is an information appliance device that generally contains a TV-tuner input and displays output connects to a television set and an external source of signal, turning the source signal into content in a form that can then be displayed on the television screen or other display device. They are used in cable... Found on http://en.wikipedia.org/wiki/Set-top_box
Also called converter boxes, these receivers convert broadcasts (analogue cable, digital cable, or HDTV) for display on a television. HDTV-ready TVs (those without a built-in HDTV tuner) must be connected to a compatible HDTV tuner set-top box in order to receive digital television programs.
Found on http://www.sony.co.uk/glossary/ShowGlossary.action?site=odw_en_GB§ionty