The “digital switchover” is the term used for the transition between analogue and digital TV broadcast.
At the moment both types of TV are being broadcast simultaneously but by 2012 the analogue broadcasts will be switched off and only digital channels will be available.
What this means in real terms is that by 2012 you will only be able to watch TV through your aerial with either an Integrated Digital Television (iDTV, basically a TV with a built-in digital Freeview tuner) or with a Freeview set-top box.
For reliable viewing of digital Freeview channels most properties will require a new aerial and cable, and in some areas almost every property will require a new aerial due to the way that the channels are broadcast.