The control of users and groups exists at the core of Red Hat Linux system administration.
Users can be either people (accounts tied to a physical user) or logical users (accounts that exist for applications so that they can perform specific tasks) . Both types of users have a User ID (which is usually unique) and Group ID.
Groups are logical expressions of organization. Groups form the foundation of tying users together and giving them permissions to read, write, or execute a given file.
Any file created is assigned a user and group when it is made. It is also assigned separate read, write, and execute permissions for the file's owner, the group assigned to the file, and any other users on that host. The user and group of a particular file, as well as the permissions on that file, can be changed by root or, to a lesser extent, by the creator of the file.
Proper management of users and groups, as well as assigning and revoking permissions, is one of the most important tasks of any system administrator.
Managing users and groups has traditionally been tedious, but Red Hat Linux provides a few tools and conventions to make users and groups easier to manage.
While you can use useradd to create a new user from the shell prompt, a popular way to manage users and groups is through redhat-config-users (see the Official Red Hat Linux Customization Guide for more information).