Not to sound like the ultimate nubie, but

[winright007]

What exactly is a cheater? I keep seeing this but I'm not sure I know exactly what it is.

[NY Jester]

It's anyone who tries to take advantage of or manipulate the rules to benefit oneself by violating rules on purpose.

[Licker4U]

Or following the rules to get listed then changing the site afterwards so it violates the rules (adding blind links, adding scripts, removing recip links, etc.)