You may know that 1080i refers to 1080 lines of vertical resolution interlaced. This means that instead of 30 frames a second, the TV displays 60 fields per second, where each field contains alternate lines of the picture. Because the fields are refreshed so quickly, your brain interprets the signal as 30 full frames per second.

720p means 720 vertical lines of progressive scan video where every frame of the signal is displayed in its entirety, giving a higher-quality, film-like appearance to the picture.

Most HDTVs or HDTV-ready TVs support either 720p or both 1080i and 720p, very few support 1080p HDTV. Part of the reason for this is that very few broadcasters want to broadcast 1080p signals. The reason for that is that 1080p requires significantly higher bandwidth than either 1080i or 720p. And for broadcasters, bandwidth is a precious commodity. Using up more bandwidth for one signal means that there is less room for other channels. So in many cases, opting for a 1080p signal would mean dropping or degrading other channels.

Is 1080p HDTV important? Well, yes and no. Not all video footage benefits greatly from progressive scan. Sports broadcasts and action movies look better at 720p than 1080i because there is lots of fast movement and the interlacing in 1080i is sometimes noticeable, reducing the quality of the picture. Movies and TV shows where there is not a great deal of fast action look better at 1080i than 720p because the higher resolution has a greater impact on quality than the progressive scan, which has little effect where there is only a small difference in the image between frames.

So, while 1080p HDTV is the best of both worlds, the cost in terms of bandwidth, is very high compared to the benefit for most people most of the time.