You use anyone or anybody in statements with negative meaning to indicate in a general way that nobody is present or involved in an action.

You use anyone or anybody in questions and conditional clauses to ask or talk about whether someone is present or doing something.

You use anyone or anybody before words which indicate the kind of person you are talking about.

You use anyone or anybody to refer to a person when you are emphasizing that it could be any person out of a very large number of people.

You use anyone who is anyone and anybody who is anybody to refer to people who are important or influential.