What is money? You might as well ask, "What is time?" It's one of those concepts we all think we understand until we really examine them. After all, we use money in its various form to buy things every day. But where does it come from? What does it represent? And most important of all, what stands behind our confidence that if we use our money to pay for something, the seller will accept it? This note answers all of those questions, and more.