You might have noticed that the Ownership & Permissions section of the Info panel enables you to specify permissions for the owner of the file, the group to which that user belongs, and others. But what is a group? Let's look at that concept briefly now.
In Mac OS X, users can be classified into many different groups so that they can access, or be excluded from accessing, certain information. There are many possible groups to choose from in the Owner and Group pop-up menus of the Ownership & Permissions section. Among them are the names for each of the user accounts on your computer, which are used to assign a file to those users. Other than those, the only groups you should be concerned with are admin and staff, which grant access to only administrative users and everyone with an account on the computer, respectively. (The other options are specialized groups that you won't need as an average user.)
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During the discussion on Mac OS X's file structure in Chapter 3, "Working with Windows, Folders, Files, and Applications" you learned that the folders at the top level of the hard drive can't be modified, which has been done to preserve system order and stability. The technical reason that even administrative users can't make these changes is that those folders are owned by another account called the root, or superuser, account. This account exists on a completely different level, one that most users of Mac OS X never need to see. Although the administrator account works for most Mac OS X system administration, the root account is much more powerful. For this reason, Apple has tried to ensure that only users who understand its power will use the root account. Mac OS X comes with the root account shut off. Because the power, and consequences of the root account are beyond the scope of this book, you may want to read " Mac OS X Unleashed ," or another in-depth book about OS X, to learn more.