“l33t” it also called LetsSpeak. Its Internet Geeks Language. See an Example.
Hi, Its Asif, How Are You?
In l33t it will look like “HI, i72 45iF, H0w rU?”

Its a way of graphic numeral presentation of alphabet.

See Details From Wiki Here

l33t Online Converter: Here
* This is great tool, it works both end, l33t->normal and normal ->l33t

l33t Details Described: Here

Interesting thing is Google has a l33t search interface. Its very interesting. See it in action: Here