A user is a database object that identifies a person who can login or connect to the database.
a role that specifies their responsibilities (see Roles (PDM))
membership in a group that specifies their rights (see Groups (PDM))
privileges that control their level of access (see Granting system privileges)
permissions that allow them to perform actions on objects (see Granting object permissions)
Not all DBMSs support the concepts of user, role and group.