Browse most of the threads on this forum. A lot has been talked about.
Check out google for Sci.Electronics FAQ tons of TV/Monitor repair tech info there.
Overscan = the process of drawing the picture outside of the visible area of the tube, "overscanning" it too tall and/or too wide. Generally some overscan is perferable (since otherwise your actual picture is too smal AND you see the hard edges of the scan lines), but too much and you of course end up with a stretched picture and clipped, especially in the rounded corners.
Tearing = the picture (typically at the top of the tube in a normal TV horizontal layout) that is angled to the right or left for the first inch or so of the screen. Caused on TV by things like Macrovision protection, but on game usualy by dry capacitors, AC line interference/dirty connections, magnetism, or other monitor problems.