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".