The moneychangers in the temple?

According to the new testament Jesus drove the money changers out of the temple. What exactly where they doing there? Moneychangers presumably mean people similar to those at international airports who exchange one currency for another for foriegn travellers, but why would they be in the temple? As far as I know the only trade in the temple precincts was the Sadducee's sale in animals for sacrifice. Also if relevent why did JC object to these moneychanger's being there.