Slangs are nothing but Informal words or expressions that are used in the spoken rather than in the written English. As they are strictly informal, you might want to use them mostly with friends, colleagues or people close to you. You'll want to refrain from using slangs with your boss, teachers or elders. It is important to also know that slangs are not unique words. They are, however, derived from other common English words or maybe other slangs too.