IRS Form 1120 is the form needed for C corporations to complete their tax returns. Form 1120S is used for S Corporations to file their tax returns. Some corporations choose to hire a CPA to file their taxes, while others choose to complete the process themselves. Businesses looking to save money by filing their own tax returns can find the process much easier using 1120 tax software. Many tax software companies offer software that you can download to your computer or complete online.