The term set-top box describes a device that connects to a television and some external source of signal, and turns the signal into content then displayed on the screen. The signal source might be a satellite dish, a cable (see cable television), a telephone line, a DSL connection, or even an ordinary VHF or UHF antenna. Content, in this context, could mean any or all of video, audio, Internet webpages, interactive games, or other possibilities.

In the United Kingdom Digital Set-top Boxes (often referred to as Digiboxes) are usually for Digital Televison through services such as Freeview a Digital Terrestrial service operated by the BBC, or through Digital satellite with [BSkyB], also with digital cable. They are used to access television as well as audio and interactive services through the "Red Button" promoted by broadcasters such as the BBC with BBCi or Sky with Sky Active.

Before cable-ready TV sets, a set-top box was used to receive analog cable TV channels and convert them to one that could be seen on a regular TV (channel 3 or 4 in North America, or channel 6 or 7 in the United Kingdom). A digital set-top box is needed for receiving digital TV broadcasts, because the vast majority of TV sets do not yet have such a tuner. In the case of direct broadcast satellite (mini-dish) systems such as SES Astra, DISH Network or DirecTV, the set-top box is an integrated receiver/decoder (IRD). In IPTV networks, the set-top box is a small computer providing two-way communications on an IP network, and decoding the video streaming media.