People who steal credit cards (usually in industrial quantities, often mag stripe data), then use those credit cards fraudulently. You get credit cards by retail theft (corrupt waitstaff, etc.), site breakins (particularly before PCI and vaulting, when every crappy ecommerce site had a bunch of card data lying around), compromises at network participants like acquirers, corrupt staff, etc.

There are whole communities of people in this space -- it tends to be fairly large and organized groups, since the actual monetization side involves printing a bunch of plastic cards and distributing them to people to make purchases (at retail).

It's related to the ATM skimming/theft/etc. world, and overlaps a lot with "traditional organized crime". Lots of Eastern European presence, too.

Carding sites and forums exist to make a market in credit cards, exploits to get more credit cards, tools to monetize them, etc.

That sounds like a lot of effort and genuine technical skill to invest in crime. If that is true, you have to wonder what motivates people to do it. Is it really profitable enough to be rationally worth the risk if you don't care about the ethics involved, or is it that the people who do it can't find legitimate employment where they live due to the state of the local economy or have personal traits that prevent them from being employed?

2-4% of all credit card transactions are fraudulent. Credit card fraud is a $200B+/year business.

It's extremely lucrative. You can learn and buy everything you need to get started on the open Internet.

1) It's incredibly profitable for some of the participants

2) Until maybe 2007, there were not a whole lot of comparably-profitable licit opportunities for Eastern Europeans. The contract marketplaces have helped. We really need to fix our immigration system in the US (and, worldwide, really) to help more.

3) A lot of the more dangerous parts are essentially blue collar crime; this is FAR safer than drugs/prostitution/human trafficking/terrorism/armed robbery/etc., so it's a clear decision for those guys.