Changing file permissions and attributes in UNIX

chmod 755 file       Changes the permissions of file to be rwx for the owner, and rx for
                    the group and the world. (7 = rwx = 111 binary. 5 = r-x = 101 binary)
chgrp user file      Makes file belong to the group user.
chown cliff file     Makes cliff the owner of file.
chown -R cliff dir   Makes cliff the owner of dir and everything in its directory tree.

You must be the owner of the file/directory or be root before you can do any of these things.