It's what you see when your videocard is pumping out game frames faster than your monitor can display them, so you wind up with the top part of the screen showing something different than the bottom part. It's easily fixed by turning VSync on in the game's settings so the picture is updated in sync with the monitor's refresh rate. You also see it a lot with shitty cell phone videos where the screen "tears" in the middle while the shooter is moving the camera left or right.
I'm not sure that's what OP is talking about, though. It's not exactly a detailed post.
An Example. You're playing MW3, and you're walking down an alley. The screen jerks slightly towards you as you're moving. Just enough of a tear to be annoying. Your explanation did it's job, but it occurs on occasion even with vsync on.