Get help at the Linux command line

For those of us who need some extra command line help (CLI):

whatis -- Display a summary of a command (rather than the entire manual)
apropos -- Display a list of commands that pertain to (are apropos to) a keyword
whereis -- Display information about the location of a command: the executable, the source code (if any), and the man pages.
which -- Display which version of a command will execute (for when there are two, or more, commands with the same name installed on the system).

via LifeHacker