From our podcast section, learn expressions here.
This section needs volunteers to work on it and who have ideas on how to develop it! So your work on this will be welcome. You could also form a team, if you like. I've got some half-baked info on smileys and Net language, in case you're interested.
American slang Listen for free, it's an audio from NPR. (You can buy the transcript, too.)
Under construction: there's more info if you visit each variety at World Englishes
While this happens, here is a definition from Wikipedia:
Slang is the use of highly informal words and expressions that are not considered standard in the speaker's dialect or language. Slang is very often colloquial; the language and dialect tend to be specific to a particular territory.
Slang terms are frequently particular to a certain subculture, such as musicians, and members of a minority. All the same, slang expressions can spread outside their original arena and become commonly understood; recent examples include "cool". While some such words eventually lose their status as slang, others continue to be considered as such by most speakers. In spite of this, the process tends to lead to their replacement by other, less well-recognised, expressions by their original users.
Slang is to be distinguished from jargon, the technical vocabulary of a particular profession, as the association of informality is not present. Moreover, jargon may not be intended to exclude non-group members from the conversation, but rather deals with technical peculiarities of a given field which require a specialized vocabulary.