To remove all the files within a directory you have to use the -r flag (r for recursive) will remove directory and all its contents. If you don't want to be asked to confirm every removal you can also use the -f flag (f for force).
as in: $ rm -rf <directory>
Be careful, be very very careful, with this command, especially if you are root. In fact unless you're very awake, very sober, and know exactly what you are doing, it's probably best not to run this command as root at all. There's a reason for david's .sig warning. Immediately after the / , with no space, you enter the directory name (as eg. rm -rf /myfiles) if you make a typo and put a space in (rm -rf / myfiles) the system will read the / *only* and operate on that....and in *nix / indicates the root directory.
So if you did enter the command david has in his .sig as root, you'd remove every file in your root directory, and all files and directories dependent on it (ie. every file on your root filesystem) without being asked to confirm anything.
As you may appreciate this could suck a little bit
Linux has no 'undo' function. Read man rm or info rm for further details and flags.
My most used commands are much as OldBob's; far and away the most used being pwd, because I'm always getting lost