Ref: http://www.dartmouth.edu/~rc/help/faq/permissions.html
Understanding file permissions on Unix: a brief tutorial
(For files on AFS fileservers, see below)
Every user on a Unix system has a unique username, and is a member of
at least one group (the primary group for that user). This group
information is held in the password file (/etc/passwd). A user can
also be a member of one or more other groups. The auxiliary group