The sudo command (substitute user do) allows a user to execute a command with the privileges and identity of another user. Many Linux systems and OS X come with sudo pre-configured allowing any user designated to be an administrative user to use the sudo command to execute commands with root authority.
The flat file /etc/sudoers is used to configure which users are allowed to execute which commands on the system.
While there are other options to the sudo command, most users will be using it to run a command with root authority, such as
sudo chgrp wheel Users
The user will be prompted for their user password before being allowed to run the command
See Apple’s Mac OS X Manual Pages for further command details.
See my other CLI articles