The dollar is only backed by faith. The fact that most people want money, and the fact that there is not enough dollars floating around out there such that everybody can have all the dollars they want is what gives money value. If we lose faith in the value of our money, then it will become worthless.
Money is simply an efficient medium of exchange .
Think about it, it is much more efficient to go to Walmart with a pocket of dollar bills than it is to go to Walmart with a truck load of chickens and then try to swap chickens for the items you need. Then Walmart has got to swap your chickens for the goods that they purchase to stock the store. This can become very time consuming and costly. Money is simply more efficient.