"Carders" are the people who buy, sell, and trade stolen credit card data online. This carding forum video provides an example of an online forum where stolen credit cards are bought and sold.
Hackers rely on a variety of techniques to obtain credit card data. One such data theft technique is wardriving, in which criminals hack into wireless networks and install spyware. Another is phishing, in which spoofed emails prompt the victim to enter account information. Phexting or smishing are similar to phishing, but with text messages instead of emails. Some hackers use keylogging software to spy on victims' PCs. Others affix devices to the faces of ATMs and gas pumps in order to skim credit and debit card data.
Wired reports an FBI agent calling himself Master Splynter was assigned to the underground and had created an entire backstory for Master Splyntr to get the criminals on the underground sites to trust him.
In the course of his dealings with "carders" (criminal hackers dealing in stolen credit cards) he developed relationships with the leaders of a particular forum. This relationship proved paramount as an attack came upon this forum jeopardizing its existence. Master Splynter convinced the forums leaders to move the forum off its current server and onto his own server that was "well hidden from law enforcement" and safe from other hackers.
Now the FBI had full control of all the traffic moving through the forum and was able to identify the credit card theft suspects, make some arrests and take down the forum.
While an accomplishment, it's only a small one as carding forums pop up every day to take this one's place. As long as credit cards as we know them are easy to compromise via skimming or hacking and anyone can make a card not present transaction over the web, credit card theft and underground forums like these will proliferate.