Check out Fractional-reserve banking --
http://en.wikipedia.org/wiki/Fractional-reserve_banking
It'll throw you for a loop, and explain what's going on.
Basically, you deposit $100 in a bank, and that bank only has to keep, let's say, 10% on reserves, or $10, and can loan out the remaining $90 -- Thereby "creating" $90 and putting it back into the economy, meaning there's $190 out there (which was only really based on $100).
Do that enough times, and you get billions of dollars as "assets", when only a fraction of that money actually exists.
