What is a bank's main source of income ?

Banks main income still comes from arbitrage gaps and such, but in our days banks income from commissions is worryingly increasing, on the expense of the the private consumer. 

Probably the fees charged for overdrafts, returned checks, and fees on checking/savings accounts that are underthe specified minimums.  Also, the interest they charge on loans, e.g. mortgages, or what those places that buy the mortgages from the bank pay to buy them.  I'd guess that a lot of income comes from the differential between what they pay investors (saving account holders, etc.) and what they can loan out that same money for to others.  Add all the many people who have accounts, and that adds up.

Just think how much they'd have if they stole 1 cent from each account each month from every customer.

Banks earn most of their money through investments, interest from loans and fees for their various services.

it came from us, simple people, from out taxes and vat. thats why home based business dont have tax and vat.

You talking about retail banks or investment bank?

retail banks make money by underwriting loans.  Clients deposit money in their saving accounts and earn a low interesting rate.  Banks lend these money to clients in mortgage, car loans...etc and charge a higher interest rate. 

Investment banks provide service and earn fees.  They are just like lawyers or other professionals.  The slightly difference is that they offer services in financial market, such as raising capital for companies, helping companies to get listed..etc.  Besides, their sales and trading department makes big money by speculating in the market too.