Leet speak, also known as leet, leetspeak, leetspeek, or hakspeak, is a way of writing words by substituting numerals or special characters for some of the English letters. "Leet" derives from "elite," which refers to hackers, who were among the first to use it and who elevated it to a sort of cult language.
In leet speak, characters and combinations of characters are chosen to resemble the letters they replace, so that the resulting word is visually similar. The word "LEET," for example, is often spelled "1337."
Here's one version of how the word "hacker" might look:
Leet is human-readable -- albeit with some difficulty -- but more problematic for a computer to decode. Leet speak can be used to bypass text parsers and to encrypt certain expressions. It can also be useful in constructing passwords that are easy for the originator to remember, but difficult to steal by means of a dictionary attack.
One of the most common, and annoying, uses of leet speak is in the dissemination of spam. Although spam filtering programs have become good at detecting leet speak in subject lines and rejecting e-mail messages that contain it, they often have less success detecting it in e-mail text, where it is abundant and may be used to get pornography or other undesirable content past filtering programs.
Leet speak is often used in chat rooms and is also favored by computer-literate children who want to conceal the content of their online conversations from parents or teachers. It is also sometimes used just for the fun of it.
Text that combines letters and numerals is sometimes called alphanumerish.