It's like the little man at the ballpark with the coinpurse. Only digital. And angry.
A digital currency exchanger is kind of a market maker who exchanges legal tender, like U.S. dollars, for a digital currency like Bitcoin. Or DisneyBux.com or whatever.
The Marker exchanges one digital currency for another. They make their money through commissions and/or fees. And in this brave-ish new world, the fees are usually high, so know what you want so you don't get nickel-and-dimed to death.