The manual page system built into Unix-like operating systems is often very helpful for looking up how to use a particular command. But what do you do if you know what you want to do, but not the command name? Or you want to look up all the commands relating to a specific keyword?
The apropos command looks through the man database and does exactly that. Give it a keyword and it will return you a list of possible matches, along with a handy description of each one.
user@galidonetworks: ~$ apropos mkdir
mkdir (1) - make directories
mkdirhier (1) - create a directory hierarchy
It is a vital tool in your command line toolbox and can often save your sanity when your memory fails you on that command name. Enjoy!