Interlacing is a technique of improving the picture quality of a video signal without consuming extra bandwidth. In an interlaced scan, only half the screen is refreshed at a time. The video signal beam skips every other line, and fills in the missing lines on the next pass.
Interlacing uses two fields to create a frame. One field contains all the odd lines in the image, the other contains all the even lines of the image. Interlacing causes a certain amount of visible flicker, but in live video it is hardly noticeable. When interlaced video is watched on a progressive monitor without deinterlacing, it exhibits 'combing' when there is movement between two fields of one frame.