Screen tearing is where two (rendered) frames more of less colide with eachother. It's not that you actually see two screen badly stitched together. But it just feels less smooth.
For the whole technical explanation see:
http://en.wikipedia.org/wiki/Screen_tearing
Maybe if you are like 15 and on a dozen Red Bulls you will actually see where it all goes wrong. For my after a day of work it just feels less smooth.