I've always wondered what sudo means when I run a Terminal command like this:
sudo shutdown -r now
sudo is an abbreviation of "super user do" and is a Linux command that allows programs to be executed as a super user (aka root user) or another user. It's basically the Linux/Mac equivalent of the runas command in Windows. Commands that can be run with sudo are defined in a file named sudoers in the /etc private folder. This file should only be edited using the visudo command.