HD is really about getting a nice resolution image on a big screen. My 27 inch sony looks great but my friends 50" rear projection tv looks like crap. Its an old Standart Def CRT TV but it always looked bad. It sacrificed image quality for size. Now with HD my 50 inch HD plasma looks awesome in HD. Looks better than anything out there.
Its the same concept as a digital camera. More megapixels does not make a 4x6 image any better no matter how many mega pixels you have. But once you begin to blow up images the megapixels count. Pictures taken w/ my camera phone look nice on my phone and like crap everywhere else
HDTV is anything over 1024X768 You multiply that and that gives you the pixel count. just under 1million
The last number is the amount of lines from top to bottom.
The first is from left to right.
So a 720 TV has 720 lines from top to bottom
A 1080 TV has 1080 lines from top to bottom
Calling a tv by this number has become kinda mainsteam even though most people don't know exactly what it means.
Also most people think 1080p is better.
I only feel its better if the TV is big enough.
55inches and above benefit from 1080p
Below that you will have a tough time seeing the benfit unless you are 4 feet from the TV. Gaming Monitors and CAD monitors do benifit from these high resolutions.. Because you sit so close. dell has a sick one out
Check out the links below they are a great place to start.