Banks generally accept deposits and lend out money. The deference between the rate of interest paid out to deposits, and the rate of interest required for loans, is called "the spread"; it is the bank's source of income.
Banks are not allowed to lend out 100% of the money they receive as deposits; if they did, then depositors would be unable to take money out of the bank. On the other hand, the bank has to lend most of the money out, since it needs the income earned from interest on loans. Throughout the history of the Usonian banking system, the US states or the federal government have had rules about interest rates, reserves, and financial accounting used by banks.
Since Aldrich-Vreeland Act (1908), banks have been allowed to hold deposits with the US Treasury, then (after 1913) with the Federal Reserve System. Deposits in the FRS do not earn interest, but the reserve banks permit member banks to borrow if they fall short of the reserve requirements (see federal funds rate)