Slang is informal words, phrases, and expressions that are not considered a standard, formal part of the speaker's language. Slang usage is often used as euphamisms, to avoid directly referring to thing that are considered taboo. It is also used to identify with one's peers, and may be used to exclude non–group members from the conversation. Slang develops as a type of short-hand to express ideas that are frequently discussed between members of a particular group. IN many cases, this can cause a barrier to communication with those not familiar with the useage of the slang terms.
Slang expressions can easily spread outside their original areas to become commonly used expressions. For example, the word "Cool" as a euphamish for the word "very good" or "excellent" originated with the jazz culture of the 30s and 40s but has since spread to become a standard part of the mainstream language. In this way, some words eventually lose their status as slang. Often, when slang spreads beyond the group that originally uses it, the original users replace it with other, less-recognized terms to maintain group integrity.
It should be noted that the word "slang" itself began as a slang term in use during the early 1700s. It has, of course, since entered the mainstream.