Form 1116 -- Foreign Tax Credit

If you owned stock in a foreign company (either outright or through a mutual fund) and it pays out a dividend, that distribution will be taxed in the foreign locale. But those dividends still get distributed to you, and so you'll owe U.S. tax on them. Because they've already been taxed overseas, you need to get "credit" for having paid already. This form will help you avoid being double taxed. Check out this story for more details.