The term 525/60 refers to the analog TV system NTSC used for years in the United States. NTSC is a system of 525 scanning lines (presented as 240 scan lines (a field), interlaced, displaying 480 total lines of visible picture (a frame), at 60 hertz. This 60 hertz scanning rate results in a full frame every 1/30 of second. The 45 scanning lines not displayed is known as the VBI (vertical blanking interval), which contains certain signals that control the TV's scanning system and other functions.
See also HDTV.
Also see Webopedia's "Did You Know...All About Digital TV . DTV and HDTV".