Linux is a multi-user and multi-tasking operating system. This means that the computer can be used by more than one user simultaneously, and the computer can also run multiple programs simultaneously.
To co-ordinate this, each user has a user name and a corresponding password. There is also a number of special users, the most important being the root user, who is the system administrator with complete access to the computer. The root user account is usually only used for system administration tasks, such as adding or removing user accounts.