ok, really simple run through. basically, standard normal tvs draw the picture every other line of the screen at a time, to simplify this we'll just say 240 lines at a time, for a total of 480 lines. This is called interlaced. You notice it if you look reaaaally close to a tv, you see black lines flickering between the pixels. 480p, the next step up from standard television, is progressive scan. It draws the entire image at once, for a smooth, brighter picture with no black lines. then you go up to 720p, which draws 720 horizontal lines on the tv, up from the 480. then finally you have 1080i. this is interlaced again, but it draws 540 lines at a time, for a total of 1080. still has faint lines if you get REALLY close up, but hardly noticeable comparitively, because the lines drawn are so much denser together on the screen than in standard tv. eventually, 1080i will become a new standard, and it is VERY nice. ultra-high res and no interlacing at all. ah and also, only 720p and up are considered "true" high definition television. most videogames these days only support 480p if they do, with some xbox games having 720p and a very few with 1080i. and dvd movies right now are usually only encoded in 480p, but im not 100% on this so someone might want to correct me.
