Virtually all HD TVs available right now are HD ready, and display pixels at least at 720p. This is great for normal DVD or Blu Ray playback and displays images perfectly well on sets of up to around 32 inches.
The next step up is to 1080p. This signifies that the TV display is capable of displaying up to 1920 horizontal pixels by 1080 vertical pixels. These pixels are refreshed or displayed progressively, which is different to the older interlaced method of displaying images which was delivered by rapidly alternating between odd and even lines on the TV display. You're unlikely to find many sets under 37 inches that can display at 1080, and you do need it that high to make full use of the quality from 1080p Blu Ray players.