Although every Unix user has a username consisting of one or more characters, inside the computer Unix represents the identity of each user by a single number: the user identifier (UID). Under most circumstances, each user is assigned his own unique ID.
Unix also uses special usernames for a variety of system functions. As with usernames associated with human users, system usernames usually have their own UIDs as well. Here are some common “users” on various versions of Unix:
Superuser account. Performs accounting and low-level system functions.
Binary owner. Has ownership of system files on some systems but doesn’t typically execute programs.
Handles some aspects of the network. This username is also associated with other utility systems, such as the print spoolers, on some versions of Unix.
Handles aspects of electronic mail. On many systems there is no mail user, and daemon is used instead.
Used (infrequently) for site visitors to access the system.
Used for anonymous FTP access.
Controls ownership of the Unix serial ports. (uucp traditionally managed the UUCP system, which is now deprecated.)
Used for Usenet news.
Used for the printer system.
Owns no files and is sometimes used as a default user for unprivileged operations.
Runs the web server.
Runs the BIND name server.
Performs unprivileged operations for the OpenSSH Secure Shell daemon.
Used for creating backups and (sometimes) ...