480p equates to 480 lines of progressive video (480 lines per frame). 480p60 refers to 60 frames per second; 480p30 refers to 30 frames per second; and 480p24 refers to 24 frames per second (film source). Usually refers to 720x480 (or 704x480) resolution.
See also HDTV.
Also see Webopedia's "Did You Know...All About Digital TV . DTV and HDTV".