Here's a good read
Screen tearing is when two (or more) "frames" are drawn on the monitor at the same time. As TJ said, it's less about the difference in FPS compared to the monitor's refresh rate, its about the relative "difference" in object positioning between the multiple frames that are being displayed.
If I quickly pan past a tree and the top half is 1/3 of my screen to the left of the bottom half across a tear intersection, that's going to be pretty noticeable, even if it's only there for a fraction of a second. Bad intersections like that "ruin" the experience.
If I'm looking at that tree while standing still, then the frame time synchronization can be even worse (further apart, say 1 second difference between the top half and bottom half of the tree, so I just dropped to 1FPS) than the previous example, but if the tree hasn't actually changed position on screen....nobody will notice!
Another (more luck-of-the-draw) aspect is where (vertically) on the monitor the two frames are intersecting. If there's an intersection right through the center of your screen where your eyes generally focus, that's going to be a lot more noticeable than when the intersection(s) migrates closer to the top/bottom of the screen.
Now, if your framerate is so high that you're drawing 3, 4, 8 frames on the monitor at a time (FCAT displays this nicely) then
technically the positioning difference across each intersection
should be smaller, but the two examples above still apply relative to on-screen movement speed. You could still throw your mouse sideways so fast that you'd get a 1/3 screen positioning difference between each of 8 tear intersections....although you'd probably need a high speed camera or still image to see it, but it's there. Before you get too excited about this, realize that it also means you've got MASSIVE overkill (aka you've spent too much on your GPU, your GPU frequency is much higher than it needs to be, or your settings are too low for that game) in relation between your GPU and monitor frequency/resolution.