Thanks to Cyrus' Blather for pointing to this article at Microsoft on computer slang:

While it's important to respect your children's privacy, understanding what your teenager's online slang means and how to decipher it is important as you help guide their online experience. While it has many nicknames, information-age slang is commonly referred to as leetspeek, or leet for short. Leet (a vernacular form of "elite") is a specific type of computer slang where a user replaces regular letters with other keyboard characters to form words phonetically - creating the digital equivalent of pig Latin with a twist of hieroglyphics.

If you've ever seen leetspeak (or 133t5p33k... or even !337$p34k) and were totally confused by it (hell, even I am sometimes), then check out this primer. They filled it with all sorts of examples for your amusement. You might actually start to understand some stuff that you've seen in online forums.