Form 1120-S is a tax form used by S corporations in the United States to report income, deductions, and other financial information to the Internal Revenue Service (IRS). S corporations are special types of corporations that pass their income, losses, and deductions directly to shareholders, avoiding double taxation. The form must be filed annually, typically by the 15th day of the third month after the end of the corporation's tax year. Shareholders then report their share of the income or loss on their personal tax returns using Schedule K-1, which is provided by the S corporation.