The user is a major concept in Linux/Unix and many other computing paradigms. The term can mean any user of a system, i.e. a person. Typically user referrs to a non-privileged user of a given computer's resources, as differentiated from the super user or root, the system administrator.
User also has a specific meaning in the system itself, that of a user account. With each account is associated a username and a serial number called the UID. Each system has a list of valid user accounts, typically stored in the /etc/passwd file but possibly obtained from some network directory. Most of these accounts (on a multiuser machine) correspond to actual people, but some of them are for administrative purposes. Occasionally, systems will have some user accounts that actually belong to groups of people.
Every process runs in the name of some user called the owner. The top command lists processes along with the owners. Files also have owners. Files and other system resources are protected and can only be accessed by particular users. Sending signals can only be done by the user or super user of the process being signalled.
Main article: User commands
useradd -m username
adds a user username and makes its homedir.