Leetspeek

Leetspeek (a.k.a. 13375p33k, leet version) is a Web language that is usually written using numbers instead of letters or built letters with other symbols. Speek is short for "speak" and leet "1337" is short for "elite"

Originally used by T3H H4><><0R5 (teh haxxors, the hackers) for not being understood, sorta code, it has been widely spreaded across the Web and is mainly used by the n00bs that desperately want to look cool or the guys that can't type properly.

An example or RA2 leetspeek is ed30t

No one uses leetspeek now, because it's old and not funny/cool anymore.