Your video card is constantly drawing new frames, one line at a time.
If vsync is off, the new frame is drawn over the previous frame. So the full frame is usually a combination of the new and previous frame . The line that's currently being drawn is the transition from the previous to new frame, and because it's disjointed it's called tearing. When the monitor refreshes, it grabs this combination of both frames - tearing and all - and displays it.
■If your FPS is lower than your refresh rate, the monitor displays each partially drawn new frame more than once (a part of the drawn frame is displayed for 2 refreshes). The tear line moves upward (advances less than a full screen each refresh).
■If your FPS exactly matches your...