CHMOD is used to change permissions of a file.

        PERMISSION      COMMAND   

         U   G   W

        rwx rwx rwx     chmod 777 filename      

        rwx rwx r-x     chmod 775 filename

        rwx r-x r-x     chmod 755 filename

        rw- rw- r--     chmod 664 filename

        rw- r-- r--     chmod 644 filename

        U = User 

        G = Group 

        W = World

        r = Readable

        w = writable

        x = executable 

        - = no permission

Here is another way of looking at it:

Permissions: 400     read by owner 040     read by group 004     read by anybody (other) 200     write by owner 020     write by group 002     write by anybody 100     execute by owner 010     execute by group 001     execute by anybody

To get a combination, just add them up. For example, to get read, write, execute by owner, read, execute, by group, and execute by anybody, you would add 400+200+100+040+010+001 to give 751.