Unofficial language (words or grammar) or interpretation of language, usually because the language is too recent or not used widespread enough to be assimilated officially. Slang exists in pretty much every language, and is part of the natural evolution of a language.
Slang is the act of shortening words, but making it sound stupid at the same time. Slang is mostly used by people ages 11-13, who want to sound "cool". Slang doesn't have to sound dumb, but it usually does.
Any non-conventional use of real words, or made up words that aren't recognized by every facet of society. Slang is often misinterpreted by people as the collection of phrases on urbandictionary.com.