Digital TV is broadcast as a digital signal through various mediums. Over the air (OTA), cable and satellite are the three methods by which most people receive digital television signals. You need some additional hardware to decode the digital signal and display it on a television or computer monitor.
This hardware can be a PCI card to install in a PC or a set-top box connected to your aerial, cable or satellite dish.
Some countries are already in the process of converting all broadcast television to a digital format. Starting in 2008 some parts of the UK will no longer have analogue TV broadcasts. There's more info on the DVB page, but note that in the US there are other variants.