Leet speak (1337 5p34k), literally elite speak or the language of the elite, is a way of transforming intelligible text in such a way that it becomes less or hardly accessible to newcomers (lamers, script kiddies, and the like).
This "language" (which is not truly a language) has its origins in bulletin board systems (BBS) in the 1980s. Legend has it that this form of writing was created to bypass text filters applied on BBS that did not allow discussions related to hacking, pornography, cracking, etc. Simply changing letters into numbers was enough to bypass the filter. The hacker group "Cult of the Dead Cow" (CDC) is said to be the first to use it, especially in their "publications."
Some later developed leet speak to the point where it became completely incomprehensible to those not accustomed to it (try level 3 on this page). This advanced form of writing can allow for basic encryption of texts, especially since translation is not made easy by the multitude of ways to transpose a letter. That's why you won't find a "decoder" on this page, only a translator.
Since that time, geek culture has become fashionable and is increasingly embraced by groups that were not previously "involved." Many terms, such as "1337," have become widely used, even appearing in advertisements, along with derivatives like "h4x0r." Many use leet speak to write their pseudonyms on online game servers or forums. Yet, few know its origin and initial purpose. Some consider the practice of ASCII art or emoticons as part of leet speak, while others believe leet speak only pertains to text. In short, if leet speak was originally used to distinguish the "elites" from the rest, it has now been adopted by many as an integral part of nerd culture.