The concept of ownership, in
UNIX, is very similar to the same concept in real life -- the
owner of a
file has the power to decide who can
access it, and to what extent. Most of the other powers conferred by ownership can be summarized to this one.
Two types of ownership exist in UNIX; each file has an owner, who is a user, as well as a group owner. Group ownership does not convey any special priveleges to the group in question; rather, it gives the owner of the file the ability to give or restrict that group's access to the file, independent of the rest of the system's users.
The owner of a file is set when the file is created; it can be changed by chown, but on many systems only the root user (the superuser) is permitted do this, because security holes are created by permitting users to give away their files to other users. (Obviously the reverse is not an issue; only the root and a file's owner can chown it.) The group owner of a file is set to the user's current group when the file is created; the user can change his/her current group with newgrp, to any group (s)he is a member of. It is also possible to change the group ownership of a file after it is created. This is done with the chgrp command, which can be used only by root and the owner of the file, not the members of the group. (The restriction on chown does not apply to chgrp.)
The permissions on the file, which can be changed with chmod, determine who has what degree of access. Read, write, and execute permission can be given to or taken from the owner, the group owner, or everyone else. Other bits store miscellaneous things such as setuid, setgid, and the sticky bit. Only the owner of the file (and root, of course) can change its permissions.