Slang is defined on Wikipedia as: "...the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially."
The Wikipedia article goes on to say that slang is usually found in "...areas of the lexicon that refer to things considered taboo (see euphemism)... and is often used to "identify with one's peers and, although it may be common among young people, it is used by people of all ages and social groups."
If you're a writer, you should be concerned with that last sentence used by people of all ages and social groups. If you're writing conversations between fictional people, sometimes, some of them will use slang expressions. Make sure your slang usage reflects the character's age and today's pop culture.
On Writing Hacks today I'm talking about using slang in dialogue so I thought I'd give you:
3 Online Slang Resources
Online Slang Dictionary
Do your research and make sure you put the right words in your characters' mouths.