Linux distinguishes between “ordinary” users and a superuser. The superuser, called root, takes care of all kinds of administrative tasks and has access to all parts of the system. Normal users lack these privileges.

All users including the superuser have their own home directories where all private data, like documents, bookmarks, or e-mail, are stored. Write access to these home directories is strictly limited to their respective owners. Folders in a home directory holding sensitive data can also be protected against read access by other users. System directories holding central configuration files or executable files can only be modified by the superuser. For more information about the Linux permission and user concept, refer to Section 19.2, “Users and Access Permissions”.

While this concept may not look very appealing at first, it adds to security. A user without root privileges cannot damage the entire system. Any damage caused is strictly limited to the user's own account and data. Any operation executed with root privileges may potentially harm the entire system. Anyone intending to harm a running Linux system must gain root privileges first. This is why it is much harder to create viruses for Linux systems. They have to overcome the root barrier first.

As well as offering different user identities for administrators and normal users, Linux supports multiple users working on one machine simultaneously. These users can connect to the system via different terminals or network connections.