Banks are financial institutions whose activity is mainly based on handling properly their customers' savings and granting loans. The viability of their business only depends on the level of trust that is perceived by their customers.
They also offer a whole range of services: They issue checkbooks and debit cards and offer you to invest and to take out insurance.

The first forms of bank appeared in the Middle-Ages. But their primary role, that of converting currencies, was framed in the 15th century.
In other words, banks met the expectations of an economic system that kept growing faster and faster until becoming global.
At first, they paid great attention to the fact of owning sufficient funds , whose financial value had to match with the sum of money deposited in their coffers by customers.
When they lent money, they needed to be completely solvent: in the case where one refund was missing, they still had a sufficient volume of liquid assets for the other customers.

Since the industrial revolution, and after WWII, banks have emphasized their role as credit terms issuers.
Banks are no longer just some sort of giant safety-deposit boxes. They offer to take out insurance, help companies to land financings and decide of their own interest rates.
Customers may ask for their money to be invested and some companies may ask via their bank to be registered on the stock exchange market.
The States have their own bank: for the United States, it is the Federal Reserve. These banks are in charge of regulating all the other banks and financing the states' budget deficits.

From the first financial crisis in 1929 to the 2008's crack, banks have been blamed for their hazardous management and their dubious operations.
They supported speculations on markets that were ready to blow up and granted toxic credit terms to unreliable customers.

In 2008, Lehman Brothers went bankrupt and left behind it a debt that amounts to 613 billion dollars...