What is and how to use Command Prompt?

Well, I'm suddenly intrested in this thingy, and want to learn what it does. If I google or youtube it, they just show how to hack things with it, which I don't want to learn. So, can anyone teach me this thingy?

A good place to get started:

http://www.bleepingcomputer.com/tutorials/tutorial76.html

Thanks a HECK!!!