Preferred and common stocks are different in two key aspects.
First, preferred stockholders have a greater claim to a company's assets and earnings. This is true during the good times when the company has excess cash and decides to distribute money in the form of dividends to its investors. In these instances when distributions are made, preferred stockholders must be paid before common stockholders. However, this claim is most important during times of insolvency when common stockholders are last in line for the company's assets. This means that when the company must liquidate and pay all creditors and bondholders, common stockholders will not receive any money until after the preferred shareholders are paid out.
Second, the dividends of preferred stocks are different from and generally greater than those of common stock. When you buy a preferred stock, you will have an idea of when to expect a dividend because they are paid at regular intervals. This is not necessarily the case for common stock, as the company's board of directors will decide whether or not to pay out a dividend. Because of this characteristic, preferred stock typically don't fluctuate as often as a company's common stock and can sometimes be classified as a fixed-income security. Adding to this fixed-income personality is the fact that the dividends are typically guaranteed, meaning that if the company does miss one, it will be required to pay it before any future dividends are paid on either stock.
To sum up: a good way to think of a preferred stock is as a security with characteristics somewhere in-between a bond and a common stock.
(To learn more about the similarities and differences of common and preferred shares, see our Stock Basics Tutorial.)