720p improved on the 480i and 480p of standard definition terrestrial television (SDTV). In TV signal specifications, like 720p, 1080i or 1080p, the numeral indicates the number of horizontal lines in the complete raster. The p in 720p stands for progressive scanning, in which each scan displays every line in the image raster sequentially from top to bottom, so that the entire image is refreshed with each scan.
The i in 480i stands for interlaced scanning, in which each scan displays alternate lines in the image raster, meaning that two complete scans are required to display the entire image. There is no interlaced equivalent to 720p.
Both 720P and 1080p increase visual quality in video converted from film, through increasing resolution and reducing the incidence of visual artifacts.