A Question About Cloud Computing

I've been researching cloud computing. I understand the concepts and ideas, but do not understand technically how it works.

Can someone help me understand how the hardware works to group together computers to pool resources? Am I asking the right question?

I just don't understand how you can pool 2 or more PC's to use its hardware resources to crunch data and present it in a timely and fashionable manner...

please help me understand it more.

thanks.