In popular terms, stock usually refers to common stock of
companies. A piece of common stock is a right to a share in the company's future earnings, and a right to one vote in the election of the
Board of Directors for the company.
The major point of stock is that it is limited liability. If you purchase stock and act as a passive investor, you do not have further responsibility beyond your initial investment. This differs from trading in margin accounts, futures, or similar enterprises where the money you pay is just a good-faith deposit.