What Are Authorizations?

Hi, im newbie here, someone could be able to help me and give mi some answers on the following questions please :

Why are authorizations used?
What are the elements of a role?
What does SAP mean by role-based authorization concept?
Who already uses or is implementing this?
What advantages does the use of roles provide?
Which steps are necessary for a user menu to be displayed for a user?

Thank you in advance