HDTV is a new global standard for the way television pictures are displayed (and therefore also recorded andbroadcast). In Europe the old colour TV briadcast standard was PAL which consists of 625 horizontal lines of 'dots' which formed the picture (in the US the standard was NTSC (525 lines) and over much of the former USSR and some of Africa SECAM was used).
The formal definition for HDTV is to have 1080 lines of data, but in practive any format with more than 720 lines of resolution is called High Deinition.
HDTV stands for High Definition Television and is a new standard that is evolving to replace the OLD PAL, NTSC or SECAM standards. All HDTV sets are widescreen, and if they have a tuner it will be digital as HDTV is only available on digital broadcasts (currnetly in the UK this is only available from Sky).