OS X Mountain Lion: About administrator accounts
An administrator is a user who can add and manage other users, install software in the Applications and Library folders, and change computer settings. The user you create when you set up OS X is an administrator.
Your computer can have more than one administrator. When you create a new user, you can make the user an administrator. You can also convert standard users to administrators.
To keep your computer secure, don’t share administrator names and passwords. Be sure to log out when you leave your computer, and use the Security & Privacy pane of System Preferences to require a password after sleep or a screen saver begins. If you leave your computer while you’re logged in, someone could sit down at your computer while you’re away and make changes using your administrator privileges.
Don’t have an administrator automatically log in when the computer starts up. If you do, someone could simply restart the computer to gain access with administrator privileges.