chmod 400 file To protect a file against accidental overwriting.
chmod 500 directory To protect yourself from accidentally removing, renaming or moving files from this directory.
chmod 600 file A private file only changeable by the user who entered this command.
chmod 644 file A publicly readable file that can only be changed by the issuing user.
chmod 660 file Users belonging to your group can change this file, others don't have any access to it at all.
chmod 700 file Protects a file against any access from other users, while the issuing user still has full access.
chmod 755 directory For files that should be readable and executable by others, but only changeable by the issuing user.
chmod 775 file Standard file sharing mode for a group.
chmod 777 file Everybody can do everything to this file.