In the process of scanning, the electron beam scans the CRT, laying down the first line of information. When the beam reaches the edge of the area defining the TV screen it is turned down to a low voltage, although it continues for a moment in the same direction. When the beam reaches the edge of the CRT, it quickly reverses direction and returns to the other side of the screen. Once it reaches the other side, the beam resumes its original direction. Its voltage is then turned back up to lay down another line. This process occurs every time lines are scanned. The duration of the lowered voltage from the end of one line to the beginning of the next is called horizontal blanking.
During the horizontal blanking the return of the electron beam from one side of the CRT to the other is called retrace.
What happens at the end of each line is similar to what happens at the end of each field. After laying down a field of information, the electron beam is turned down to a low voltage before it retraces to the top of the image. Once in position at the top of the screen, the beams voltage is turned back up and it starts scanning a new field. The time that the beams voltage is turned down until it is turned back up again is called vertical blanking or the vertical interval. When the elecytron beam is retracing back to the beginning of a new field it is called vertical sync.