What is the main difference between a stock and a bond?

A bond is a debt instrument. The company or government issuing it borrows your money and pays you a fixed amount of money for the use of the loan you have made available to the company or government. The selling price is usually what the face value of the bond is, but this can vary according to interest rates determined by the Federal Reserve.

A stock is ownership. You own a fraction of the company you've invested in. Sometimes a company pays a dividend. That means that the company has excess funds and decides to pay its shareholders a fraction of what the company brings in.  When you buy a stock, you expect to sell it at a higher price than what you bought it at. That's called a capital gain. It's another source of income.