Form 1120 is the U.S. federal income tax return used by corporations to report their income, gains, losses, deductions, and credits. It is filed annually with the Internal Revenue Service (IRS) and is essential for determining the corporation's tax liability. Corporations must complete Form 1120 to comply with federal tax laws, and it includes various sections for detailing financial information. The form is typically due on the 15th day of the fourth month after the end of the corporation's tax year, and extensions can be requested if more time is needed to file.