OS X Lion: About administrator accounts
An administrator account can create and manage other accounts, install software in the Applications and Library folders, and change computer settings. The account you create when you set up Mac OS X is your computer’s administrator account.
You can create multiple administrator accounts on your computer. When you create a new user account, you can make the account an administrator account. You can also convert non-administrator accounts to administrator accounts. To learn how to set up administrator accounts, see:
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 account automatically log in when the computer starts up. If you do, someone could simply restart the computer to gain access with administrator privileges.