Form 1120S is a tax form used by S corporations to report their income, deductions, credits, and other relevant information to the Internal Revenue Service (IRS). S corporations are a type of corporation that elect to pass corporate income, losses, deductions, and credits through to their shareholders for federal tax purposes. This means that the corporation itself does not pay federal taxes. Instead, shareholders report their share of the corporation's income or loss on their individual tax returns. The Form 1120S is used to calculate the taxes owed by the S corporation itself and to provide information for each shareholder's individual tax return.