MMO games such as World Of Warcraft have their own economies, as one would expect from a virtual world. The surprising practice that has sprung up ever since the creation of MMOs is that of gold selling, or gold farming as it is commonly called. The premise is simple; As players work their way through the game, they earn items, weapons, and money in the form of gold. This gold can then be used to purchase better armor, weapons, and other items available in the game. Not too long after the MMO industry began (specifically with the game Ultima Online and rise of Ebay's popularity), gold selling companies began to pop up all over the place. They employ workers (usually at very low wages) to work 10 - 12 hours playing the game and accumulating gold. This gold is then posted on Ebay or notices are posted within the game itself advertising the gold for sale. The buyer transfers real money to the company - via Paypal or some other mechanism - and in return the "virtual gold" is transferred to their character in the game.
Gold sellers also provide other services, such as power leveling characters. Basically this means you allow an employee of the company to play your character (via your username and password) for some time until the desired level is reached, and you pay real money for this service. Another service gold sellers provide is the selling of accounts. For example, if you have an account that has ten characters at the highest level possible, you may not wish to play the game any longer. Imagine if you can sell this account to someone else however; they get characters that already have the levels and gear, and you get real money for the time you spent playing the game. The gold sellers act as the middleman, processing the transactions for a fee.