How are TV ratings worked out?

A carefully selected sample of households are provided with elecronics which monitors which channel they are watching. Each member of the household presses a button to indicate when they are actually in front of the box. The Broadcaster's Audience Research Board collates this data, weights it for socio-demographic reasons and publishes it overnight to all broadcasters and advertisers. For a full explanation, see http://www.barb.co.uk/about/.