The jargon file is a dictionary of Hacker slang. Here are the first few paragraphs from the introduction:

This document is a collection of slang terms used by various subcultures of computer hackers. Though some technical material is included for background and flavor, it is not a technical dictionary; what we describe here is the language hackers use among themselves for fun, social communication, and technical debate.

The “hacker culture” is actually a loosely networked collection of subcultures that is nevertheless conscious of some important shared experiences, shared roots, and shared values. It has its own myths, heroes, villains, folk epics, in-jokes, taboos, and dreams. Because hackers as a group are particularly creative people who define themselves partly by rejection of “normal” values and working habits, it has unusually rich and conscious traditions for an intentional culture less than 40 years old.

As usual with slang, the special vocabulary of hackers helps hold their culture together – it helps hackers recognize each other's places in the community and expresses shared values and experiences. Also as usual, not knowing the slang (or using it inappropriately) defines one as an outsider, a mundane, or (worst of all in hackish vocabulary) possibly even a suit. All human cultures use slang in this threefold way – as a tool of communication, and of inclusion, and of exclusion.

Note: for hundreds of years (until sometime in early 2003), the jargon file was located at <http://www.tuxedo.org/esr/jargon/>. Then, all of ESR’s content moved to catb.org, breaking millions of links. Unsubstantiated word has it that ESR wanted to become owner of tuxedo.org but the owner refused; so ESR moved all his content to a new domain.