"One of the things that makes Unix different from Windows
are file ownership and permissions."
"All files on a Unix system are owned by a user and a
group. Users own files they create, and root can change the
ownership of files through the "chown" utility. A file's group is
set to the primary group of the user that creates it initially.
Since users can belong to more than one group (and often do), a
user can change the group of any file they own with the "chgrp"
utility to any group they belong to. You can use the "groups"
utility to list what groups you (and others) belong to."
"File permissions control who can do what to a file.
They're divided into four sections. Extended permissions are dealt
with in another part of this NHF. User permissions control what the
user who owns a file can do to it. Group permissions control what
members of a group that owns a file can do to it. World or other
permissions control what everyone else can do to the file."