Leet, LeetSpeak, L337, 1337, 31337 is a language used on the Internet by Elite Programmers, Hackers, and Gamers.
The basis of which is to replace letters with numbers and other characters that look graphically similar. There are many forms that range from a simple substitution of letters to numbers upto a very complex language using various character combination to represent every letter. Anything is valid as long as the Elite or insiders can understand it.
The main reasons for Leet are to confuse and obscure what is being said to new users (noobs) and other outsiders, to beat and filters and censoring scanners, and often to use as a substitute for your name or moniker on large internet sites like Gmail where all the typical names are already taken.