Shares of common stock allow investors to share in a company's success over time, which is why they can make great long-term investments.
In general, common stock comes with the right to vote for corporate directors, as well as the right to vote on policy changes and stock splits. There are a few exceptions to this rule, but in most cases, you'll have voting rights.
Some companies choose to distribute some of the profits on their balance sheet to common stockholders in the form of dividends, and each common stockholder is entitled to a proportional share. But this isn't a requirement.
Common stock versus preferred stock
The other main type of stock is called preferred stock and works a bit differently. The main difference is that preferred stock has a fixed, guaranteed dividend, while common stock dividends can change over time or even be discontinued. For this reason, the share prices of preferred stocks generally don't fluctuate as much as common stock.