Excerpt from the TechLore article "What Does 1080p Do For Me?"

The ATSC standard allows for two entirely different resolutions to be considered "high-definition". One of these resolutions is the 720p format, which as an actual resolution of 720x1280 in a progressive format. Progressive means that each field contains a complete picture. The 1080i format has an actual resolution of 1080x1920 (over two times the image density), in an interlaced format. Interlaced means that each complete frame is divided between two fields, one with all the odd numbered lines (line 1, 3, 5, 7, etc.) and the other with all the even (2, 4, 6, 8, etc.). When played back at 60 cycles per second, it looks like a complete 1080x1920 picture.

To keep it simple, nearly all past generation fixed pixel product has a native resolution at or near 720p. This means that any resolution less than 720x1280 won't take up the entire screen, so these images must be digitally processed so they fit a 720x1280 space. Processing signals up sometimes adds digital artifacts and other picture anomalies.

The same is true for images of higher resolution, except going down reduces the image density of the picture. Simply put, 720x1280 native TVs can display 921,600 pixels in the correct shape. A complete 1080i signal has 2,073,600 pixels. Even a single field of an interlaced 1080i picture has more image density than an entire 720p frame. In order to properly display a 1080i signal on a 720p TV, 1,152,000 pixels must be eliminated to properly fit. That's right, CSI: and all other 1080i broadcasts look half as good as they should on a 720p TV.

A native 1080p TV will still have to process signals lower than 1080p, but it will not have to reduce image quality on any signal it receives (at least for the forseeable future). A 1080i signal will need to be processed before it can be displayed as a progressive signal, but the resolution will not have to be reduced.

The move to 1080p TVs will not be an overnight switch; 720p products will still be around for a few more years. However, going forward, these products will sell for a lower price than the 1080p variety, bringing these newer technologies in the price range of nearly all consumers. It's important to remember that the difference between 720p and 1080p TVs will not be obvious at resolutions lower than 720p, but anything higher will show better on a 1080p display.

Read the full article at TechLore: "What Does 1080p Do For Me?"