Many different kinds. Some just receive money and put out BTC (to mobile or QR recipt) others you can sell your BTC to.. What kind of ATM are you looking for?
I'm living in Greece and right now with the illegal capital controls over our country the whole system is f$%^$d up and I'm trying to understand how the technology works because we are in chains and not be able to pay nothing outside the country. I also noticed that we have a lot of scammers everywhere trying to make Btc on locabitcoins and other sites and it got me really curious to check out if we can find some solution to help myself and more people like me to buy some bitcoins without the hassle.
It's disgusting to see how people are profiting from other people's misery. The world has really become a f**ked up place.
As I know, ATMs are working just like regular Fiat ATMs. They take your money bills, and give you your bitcoins. Of course, if you pay with your credit card, then these ATMs are connected with their companies over the Internet and you are actually exchanging virtual Fiat to virtual BTCs.
Generally for smaller amounts you don't need your IDs scanned, etc. For bigger, you need to be verified.
Now this is how Bitcoin ATMs work in Europe, in general. I am not sure what were they obliged to do in Greece. Maybe someone can share this here with us, that has experience with operating Greek Bitcoin ATMs.