Bitcoin is an online phenomenon. Bitcoin is a program that acts like a currency. The program allows people to earn, trade, and sell these digital coins. The program only allows for a maximum of 21 million to exist. But how to get a Bitcoin? They can be earned by reviewing a portion of the exchange log. In the real world, whenever a business has any exchanges, they must record it for review by a third party at a later date to ensure against fraud. In Bitcoin, there is no third party, outside of the system. Fellow Bitcoin owners can read the exchange log, as it is public. This is called mining.