Leet, usually written as "1337" in Leetspeak, is an online culture and/or attitude, as well as a language code, among the Internet population. The word itself is derived phonetically from the word "elite", and is a cipher, or cryptic form of spelling replacing letters with numbers, symbols, and other letters that look or sound alike. Leetspeak was first used on Bulletin Board Systems, and then later adopted by users of Online Multiplayer Games and other Internet communities.
Spelling variation does not always follow a set convention. The same word may be spelled differently by different people, or even by the same person to confuse others even more. This is symptomatic of the desire or affected desire to elude comprehension by others unfamiliar with the foreign art form.
Many consider it a pointless affectation, and as it has become widely used it is less useful as a way of showing membership of an "elite" group. It is nonetheless a cultural phenomenon well-known amongst hackers and many other Internet users, especially gamers.