480p refers to progressive video encoded in frames 480 pixels tall. 480p is generally used for video from film sources, as they're natively progressive. It can also consist of interlaced video that's been deinterlaced for a number of possible reasons. Most 480p video comes from either DVD or deinterlaced TV captures.
In addition to captured NTSC, other common formats using 480p video are Standard Definition ATSC, EDTV, DVD, some DV, and SVCD. 480p is most suitable for video formatted to play on a progressive scan Digital TV (DTV) of some kind.
Regardless of the format it's encoded in, or exact resolution of a 480p video file, they generally have a framerate of 23.976fps, which is intended to be used with pulldown flags to create a file that plays at the standard NTSC framerate of 29.97fps. In rare cases it may be 24fps instead.