Erm, you are joking right? You do know what the command line is & were just being sarcastic, right? RIGHT?
Well, i'll tell you what it is

Before windows, most PC's used a system called MS Dos, it was a Command Line Interface to the PC, meaning that, instead of a graphical interface, you had a blank screen with a cursor on it.
For starting programs you had to type in the name of the program, for copying files you had to type in a command, for changing & viewing folders you had to type in commands...
Even now, with Windows XP, it's still there, and if you examine the icons for programs you'll see the command line for that program.
Cheers,