Sometimes called "zipper effect," dot crawl refers to a specific image artifact that is a result of the composite video system. Dot crawl may be seen on TV news, for example, when a picture appears over the anchorperson's shoulder, or when some text appears on top of the video clip. If you look closely, along the edges of the picture, or the text that has been overlaid, you'll notice some jaggies rolling up or down.
Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video. It consists of animated checkerboard patterns which appear along vertical color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.