The federal government invested $45 billion in Citigroup late in 2008 in an attempt to stabilize the company and avoid a collapse of one of the largest banks in the world. In return for its investment, the U.S. Treasury received preferred stock in the company.

On February 27, 2009, Citigroup announced a plan to convert preferred shares to common stock. The U.S. Treasury has agreed to participate by matching the amount converted by private investors, up to $25 billion. If the full $25 billion is converted, the U.S. government will own 40% of the company. This is a stock for stock deal; no additional cash will be invested as part of the transaction.The Washington Post: U.S. Plans Bigger Stake in Citigroup (February 27, 2009)

Common stockholders have an ownership interest in a company, and may receive dividends at the discretion of the company's board of directors. Common stockholders are the last ones paid back in the case of liquidation. Investopedia: Common Stock

Investors who have provided loans (debt financing) to a company have no ownership interest and no voting rights. They receive interest payments and are the investors with the highest claim on the company's assets in case of liquidation.

Preferred stock shares features of both debt and common stock. Preferred shareholders have an ownership interest (like common stock), but generally have no voting rights. They receive guaranteed payments (like debt) in the form of dividends. In the case of liquidation, preferred stock comes after debt, but before common stock, when assets are distributed.Investopedia: Preferred Stock