1337 or lEET means "eleet" or rather "elite" and is mainly used for expert users/hackers on the Internet and is as old as the internet itself. 1337 actually is the language used by "eleet" people which substitutes alphabets from english language with similar looking numbers and ASCII symbols. There is also no need to follow the grammar when you are writing in leet.
In fact there is a firefox extension that allows you to write in leet. You can get it from https://addons.mozilla.org/en-US/firefox/addon/770 Apart from leet, it also allows a user to write in Hex, Binary, Morse and lots more languages.
Even Google has a leet version which can be reached at:
For more information on leet, you can have a look at following sites: