I was looking at various lists of MMO games etc the other day when I noticed that a lot of them claim to be entirely free, but then accept payment by microtransaction. In computer security, etc, microtransactions are very small amounts of money transfered in real time for services (ie for torrents and traffic routing, though that's not actually implemented in public p2p networks)
What are microtransactions with reference to games, how much do they cost, and why are they important in 'free' games?