There are basically three level of TV quality:
1) Analogue
2) Digital
3) HD (High Definition)
1) Analogue is what we all had for years with just the choice of 5 channels (BB1, BBC2, ITV, Ch4, Five). The Analogue signal will be turned off over the next five years.
2) Digital is what you get from a Freeview box (and also from Sky and cable).
If you buy a Freeview box, and plug it into any Analogue TV you can watch digital TV.
Some modern TVs come with a Freeview box built in, usually this is called IDTV (Integrated Digital TV)
3) High Definition. This is the next level up from Digital, with more lines on the screen giving you a better picture.
HD is still in its early days. Sky currently put out some channels in HD but you need a special HD "box". I think the cable companies may also be doing some HD.
HD is NOT available on Freeview and will not be for a while.
So it is important to note
BUYING A HD TV DOES NOT (IN ITSELF) GIVE YOU HIGH DEFINITION PICTURES.
To be honest I think the TV industry has conned us all these last few months pushing "HD ready" TVs, when most people will not be watching HD for 2 or 3 years, maybe more.
If all you want is Digital TV then buy a freeview box.