From LQWiki

UserName

A person who uses a computer is called the user. (Fairly obvious, isn't it.) Many systems these days have heightened security, requiring the users to login via a password. However, not all users are equal. There is the root or administrator user that has complete control of the machine, and there may be other users who need to use the machine. Each user has a user name that, teamed with a password allows them to log in to the machine with their own preferences.

A user name can be anything, but in large networks, usernames are often specified by your name- Joe Bloggs might have a user name of jbloggs for example.

Users are also an added form of security- though a person has administrative priveledges and knows the root password, they will often have their own lesser-preveledged user account to use.

Add a user

If you want to add a user whatever, use

useradd -m whatever