A nifty Unix command that gives you a short description of a given system command. From the man page of GNU whatis:


NAME
       whatis - search the whatis database for complete words.

SYNOPSIS
       whatis keyword ...

DESCRIPTION
       whatis  searches  a set of database files containing short
       descriptions of system commands for keywords and  displays
       the  result  on  the  standard output.  Only complete word
       matches are displayed.

       The  whatis  database  is  created   using   the   command
       /usr/sbin/makewhatis.

SEE ALSO
       apropos(1), man(1).