HOW TO: keep my Mac safe from other users
When you initially set up your Mac, OS X creates a single user account for you. That account includes administrative rights, which give you the authority to install, change, or delete anything on the computer. Using that administrator account as your normal, day-to-day login account can be risky.
First, you make it easier to mistakenly change or delete something crucial to your computer’s operation. And second, you open a potential security hole: if you step away from your computer without logging out, someone else will have complete access to your Mac’s data and settings. So the safest course is to set up a second user account, without administrative privileges, and use that as your main day-to-day account. When you need to install software or perform some other administrative tasks, you can still log in to the administrator account. Macworld: Feature: I will keep my Mac safe from other users