The type of tax return form you use when you file your income taxes is based on the way your business is organized. Sole proprietorships file different forms than partnerships, for example. Here are the tax returns you should file based on your form of business organization.
Sole proprietorships and one-member Limited Liability Companies (LLC): IRS Form 1040 along with Schedule A and Schedule C.
Partnerships and LLC's with more than one member: IRS Form 1065
C-Corporations: IRS Form 1120
S-Corporations: IRS Form 1120S with IRS Form 1040 and accompanying schedules for individual shareholders