A rift in your visual experience
Take a look at this photo. See that horizontal break halfway down? That break is called a “screen tear.” You’d never tolerate it in your still photographs, but screen tears are a constant torment for PC gamers. They come and go in a flash, but that’s enough to annoy.
Two sides out of sync
The problem is that your content and your monitor are rarely in complete sync. While computer monitors are refreshed at a constant rate, game frame rates are sporadic – the computer draws frames as fast as it can, and that varies constantly. Meanwhile, the display is updating at a steady rate, usually 60Hz (frames per second). That’s what causes the screen to "tear" – the monitor is being fed a new frame before it has finished drawing the last one.