There are currently three different types of flat screen on the market:
LCD - Plasma - Rear Projection.
Good explanation with pros and cons here:
http://www.which.co.uk/reports_and_campaigns/a udio_visual/reports/television/watching/lcd_an d_plasma_tvs/lcd_plasma_tvs_essential_guide_57 4_103930_9.jsp
HD is high definition tv and all of the above may or may not be HD. There are many more pixels to give a 'better' picture, but even if you have a HD tv you will not HD tv unless you have an additional HD box and the programme is broadcast in HD. Think of it as a colour tv with a black and white programme.
At the moment only Sky and Virgin are broadcasting some, not all, HDtv programmes and there is an additional monthly cost to receive it.
Some HDtvs do not have a brilliant picture when showing non-hd broadcasts.
At the moment, the best picture (apart from HDtv) is still with the traditional CRT televisions - even with a very large screen the prices are very much cheaper than a flat screen.
Unless you need to replace your tv now, I would suggest waiting at least 6 months - prices are falling, and will continue to fall.