In addition to letting you decide who can log in to your computer, Mac OS X enables you to control who can interact with your files. If you create a file while you're logged in to your account, you own that file. Without your password, other users can be prevented from accessing your folders and files in any way; they can neither read nor alter your files and folders. For example, the folders in the home folder created for each user have some of these restrictions set by default.
Changing privileges in a file or folder is done though the Info panel of the Finder. These are the steps to use this panel: