OSXFAQ - Technical News and Support for Mac OS X

Users, Groups, and Permissions form the foundation of Unix's multi-user view of the file system. Users and groups identify you, the files that you own, and the files that you share. Permissions mark out your personal 'turf' and the extent of your access to remainder of the file system - that is the files you can see and change, and the programs that you can run. Permissions are applied individually to each file and define who can read, write, and execute it.