STICKY DIRECTORIES
       When the sticky bit is set on a directory, files  in  that
       directory may be unlinked or renamed only by root or their
       owner.  Without the sticky bit, anyone able  to  write  to
       the  directory can delete or rename files.  The sticky bit
       is commonly found on directories, such as /tmp,  that  are
       world-writable.



Its how the /tmp and /var/spool/mail are set. 
Some place mentioned setting setgid (g+s).

drwxrwxrwt  3 root mail 16384 Aug  2 06:01 /var/spool/mail/
-rw-rw----  1 mehta mail 548 Aug  2 03:43 /var/spool/mail/mehta
drwxrwxrwt  4 root root 4096 Aug  2 06:00 /tmp/


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file permissions : Operation Effect on a file : Effect on a directory
read : Read the contents of the file : Find out what files are in the directory, e.g. Ls
write : Delete the file or add something to the file : Be able to create or remove a file from the directory
execute : Be able to run a file/program : Be able to access a file within a directory