You can use credit with Bitcoin for a variety of purposes, including making and accepting loans, establishing reputations, transferring value, and more.
Any time a financial transaction takes place which leaves one party more obliged than the other, the obliged party is extending credit to the obliging party. This allows groups of people and organizations to cooperate even when their resources and needs are spread out over time, and can be a highly valuable activity.
You can of course always lend or borrow Bitcoins casually with your friends, but when interacting with more distant social connections or for larger amounts, you may want to make use of a credit service. Such services can help you keep track of credit balances, interest rates and amounts, and other figures, while providing many useful features such as credit score reports or even credit exchanges using Ripple routing.
A number of free and open services allow you to manage Bitcoin credit. These include: