Most people consider business to be fairly straightforward, that is to say, if the cost of your product is $6 and you sell that product for $10, you have made a profit of $4. Of course, they have failed to take into account such things as rent, labor, equipment, overhead, and so on. In any event when a business successfully turns a profit we believe that we have seen the best of capitalism, with the invisible hand of the marketplace benefiting all parties involved. However, there is one fundamental component of business that is sometimes loss in all that activity, which is the medium of money, itself, which is virtually the only constant between one business type to another, and from one entity to another.
The fundamental thing about money is that you and I are not allowed to issue our own currency, and that the currency which we utilize on a daily basis isn't even government issued currency in the first place. If we take a careful look at our actual money, the dollar bill, states that it is a "Federal Reserve Note", and while that might imply that must be government issued money, since federal and government seem to go together, the fact of the matter is the Federal Reserve is actually a private consortium of banks that lends money to the American government.
As you might imagine, the issuer of our currency, has massive advantages over those that have to utilize the currency in order to conduct their day-to-day business. For instance, the stability of the value of money is not constant, so that those that are more aware of possible inflation or possible deflation, of interest rate increases or decreases, will be ever able to take advantage of this inside knowledge to their aggrandizement. Further to this, our banking system is able to take advantage of rules made for their benefit, for instance, the banks are allowed to leverage their deposits on file, so as to create loans and to circulate money, so that if there are $100 million in deposits at said bank, they have the ability and the power, subject to certain conditions, to loan or to invest 20x that amount, making it fairly straightforward for banking agencies to mint themselves a very nice profit.
Americans are fortunate too, because the dollar represents the reserve currency to the world, that is to say, because of our size and wealth, the dollar is the de facto standard for financial transactions throughout the world. This means, that most financial transactions are conducted in dollars, or subject to dollar conversion, or loaned in dollars, and ultimately measured in dollars, to which these banks can benefit and make money by the charging of various fees, conveniences, and services for providing this liquidity to foreign banks.
All of the foregoing essentially means, that banks, despite their protestations to the opposite, in essence make their money off of the labor of other people and companies, by on the one hand being that medium of exchange and subjecting those that must utilize money to conduct business a fee(s), and at the same time "gaming" the system, so that they can maximize their profits at the general expense of the public. Unfortunately, making matters ever worse for the general public, when certain member banks have a very bad day at the office, they are considered to be "too big to fail" which translated means that the public is stuck paying for the banks ill-advised gambles that have failed.