Despite what many people think,
leet speak is not when you make letters with non-letter characters, like
| /\/\ |_33+ 2 |)00|)!!!
Though the non-letter characters used to make letters method is used occasionally, it is primarily made up of numbers acting as letters, like so.
y0 j00!!111 u c
m3 0wnz0rz dat n00b v3r1ly ?? lolololololololo
Note that j00 is often used in place of "you" and many words end with a
0rz or a z. Also, letters aren't ALWAYS used, just when someone feels like being "l337".
Leet Speak originated as a way for getting vulgar words (such as "u r
fuxx0rz d00d!!) through filters in chat rooms and games.