HDTV is actually part of the DTV (Digital Television) specifications, which has many different video resolutions. The two main resolutions to be concerned about are 720p and 1080i. The "p" means progressive and "i" mean interlaced. In both resolutions, every second has 60 frames of video. Progressive resolution puts 60 full frames on the screen every second. Interlaced resolution puts 30 frames of only odd lines and then 30 frames of only even lines up every second. Some people complain about the flicker produced by interlaced video.
The 720p video resolution is 1280 X 720 pixels, which gives 921,600 total pixels and the 1080i video resolution is 1920 X 1080, which gives a whopping 2,073,000 pixels.
Which resolution is better? Beauty is in the eye of the beholder, and future battles may be fought over this question. Some people like the slightly more "stable" picture of 720p, while others prefer the greater resolution of 1080i. The best way to figure this out is to run down to your local TV store and see for yourself.
Just as your CDs sound better than your old audiocassette tapes, HDTV's digital audio signal sounds better than standard television's analog sound. Also, some HDTV programs include Dolby Digital 5.1 surround sound. Of course this means you need 5.1 speakers to take advantage of the better sound.