Aren't = are not. Used when the subject is multiple items/plural. "They aren't nice people."
Isn't = is not. Used when the subject is one item/singular. "He isn't a nice person."
Don't = do not. Multiple items/plural. "They don't come here often."
Doesn't = does not. One item/singular. "He doesn't come here often."
"Don't" is also used to command someone or some people to not perform an action.