What are permissions? What are roles? What are users?

What is a user ?
For example, I'm Chen yegui
What is a character ？ For example, the identity of system administrator .
What are permissions ？
Deleting a log is a log management permission , Adding a user is a user management permission
Such as can be
The relationship between them is ：
The system administrator has log management permission to delete logs , role = jurisdiction
Finally, the system administrator assigns a value to chenyegui , user = role