Find out what a command does

At the command line, run:

whatis <command>

This will give you a one-line summary of a command.

If whatis doesn't work, the whatis database may not be built yet. To build the whatis database, enter this:

/usr/sbin/makewhatis

I can't remember the name of the command I need

The apropos command searches a database of commands, using keywords:

apropos <keyword>

Note that your keyword might not work. So try different keywords until it does!

If apropos does not work, you probably need to rebuild the whatis database with this command:

/usr/sbin/makewhatis