http://en.wikipedia.org/wiki/Credit_card_fraud#Skimming - Wikipedia Says that the Lebanese Loop just retains the card which isnt technically true as the things have seriously envolved now.
The biggest fraud I heard of was where a gang hired 5 empty shops in London and installed 5 fake card machines. You put you card in and it scaned it, you then entered your pin number which it stored. The machine would then tell the person that it was unable to give cash at present so the person removed the card and went to another machine.
They did it in high tourists areas and had the machines in place for over a month retrieving tens of thousands of card details and pin numbers.