Every item on your computer belongs to, or is owned by, an account. In addition to being owned by a particular user account, every item on your computer carries with it a set of permissions that control which user accounts can access it and what kind of access they have. Permissions control who can do what to which files, folders, and disks. In this excerpt from Take Control of Permissions in Mac OS X, Brian Tanaka details ways to ensure your permissions are in order.