HDTV stands for high-definition television. As the name implies, it's a method of transmitting and receiving signals which gives a much clearer picture.
HDTV signals can only be broadcast over a digital network but digital televisions don't have to be able to receive HDTV.
Between 2008 and 2012, the older analogue television transmitters will gradually be switched off. This means that viewers without satellite or cable services will have to use a Freeview box or purchase a digital TV which incorporates a Freeview tuner.
Viewers who wish to be able to get the highest quality picture (whether via terrestrial or satellite transmissions or through cable services) will have the option to buy a digital set which is compatible with HDTV but, unlike the actual switch to digital services, this will not be compulsory.
If you're old enough to remember it (or you know enough about TV history to understand it), I'll give you an analogy.
Many years ago, everyone owned black-and-white TV sets which worked on a 405-line VHF system. Then along came the switchover to the 625-line UHF system (which offered the option of colour). People didn't have a choice about switching to 625 line TVs. After a changeover period, the 405-line transmitters were turned off, so everyone had to get a new TV. They did have a choice, however, about whether to get a colour set. Initially, many people just switched to a 625-line b-&-w set. Colour was (and indeed remains) an option but, over the years, it's become standard in most homes
The same thing is happening with digital TV and HDTV. Eventually, viewers will have to switch to digital TV but HDTV will be an option. Just like colour, however, it's likely that the optional service will eventually become standard in most homes.
Chris