Common stock is a piece of ownership into a company and is reported on the company's Balance Sheet. Holders of common stock have voting rights for specific aspects of the company like voting for a board of directors and/or company policies.
Common stock is what gives investors ownership into a company, the more stock one has, the more ownership they have in a company. One thing to note is that in the event of bankruptcy, common stock holders are usually the last to be paid out. Usually after preferred stock holders, creditors, and any bond holders.