The term Glass–Steagall Act is most often used to refer to four provisions of the Banking Act of 1933 that limited commercial bank securities activities and affiliations between commercial banks and securities firms. Starting in the early 1960s federal banking regulators interpreted these provisions to permit commercial banks and especially commercial bank affiliates to engage in an expanding list and volume of securities activities.
Basically here is it in a nutshell. When you deposit your money in the bank, lets say you deposit 100,000 dollars. The FDIC ensures upto 250,000 dollars in bank accounts, so that money is protected. A bank will take that money and wager it on the stock market. If they loose the money, the federal government pays it back... all of it.... WITH YOUR TAX DOLLARS!