A stock is a type of investment security that represents ownership in a company.
Companies issue shares of stock as their primary means of raising capital.
The value of these shares will normally fluctuate depending on the company's performance. If the company performs well or has the prospect of performing well the stock value will generally go up. This is one way to earn a return on your investment. There is a risk that the company will not perform to expectations, therefore the value of the stock could go down causing the shareholder to lose money on the investment. The other way to earn a return from investing in stocks would be if the company pays dividends from its profits.