Users can be either people, meaning accounts tied to physical users, or logical users, meaning accounts that exist for specific applications to use. Groups are logical expressions of organization. Groups tie users together, giving them permissions to read, write, or execute files.
When a file is created, it is assigned a user and group. It is also assigned separate read, write, and execute permissions for the file's owner, group, and everyone else. The user and group of a particular file, as well as the access permissions on the file, can be changed by root or, in most cases, by the creator of the file. (See Chapter 8 for more information on permissions.)
This chapter shows you how to manage users and groups with the User Manager tool in Red Hat Linux and describes the importance of shadow passwords.