The term 625/50 refers to PAL/SECAM, the analog TV system used in Europe and other countries. It's system of 625 scanning lines (presented as 265 scan lines (a field), interlaced, displaying 580 total lines of visible picture (a frame), at 50 hertz. This 50 hertz scanning rate results in a full frame every 1/25 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".