Every user account is set as a standard user account, even the administrator account on a computer. This action prevents any user including the administrator from remaining logged on and unknown programs to run in the background as the administrator with administrative privileges.
So when you create an account and make him/her an administrator, it give his account the privilege to elevate privileges for administrative tasks. He simply has to consent that he is the one requesting the elevating privileges. This is behavior of the UAC with someone who is part of the administrators group.
If a user isn't a member of the administrators group, then if he attempts a task that requires administrative privileges, he will have to provide credentials from a user who is a member of the administrators group. This is the behavior of the UAC with someone that is a standard user account.