Facebook Credits is the name of the virtual currency used on Facebook. Credits can be used in all Facebook applications and games to level up or power up your character. Credits are also used for purchasing virtual goods and items. The standard way to get Facebook credits is to pay for them with real world money, using a credit card, PayPal, Western Union or other local payment methods.
There are other ways to earn credits online. Some of these methods are paid and some are free, but all require you to do something in exchange for credits.