The su (substitute user) command makes it possible to change a login session’s owner (i.e., the user who originally created that session by logging on to the system) without the owner having to first log out of that session.
Although su can be used to change the ownership of a session to any user, it is most commonly employed to change the ownership from an ordinary user to the root (i.e., administrative) user, thereby providing access to all parts of and all commands on the computer or system. For this reason, it is often referred to (although somewhat inaccurately) as the superuser command. It is also sometimes called the switch user command.
