What is the difference between Form 1120 and Form 1120-S?

Form 1120 is the tax return for C Corporations, which pay corporate income tax at the entity level. Form 1120-S is the return for S Corporations, which are pass-through entities. The income reported on Form 1120-S flows through to each shareholder's personal return via Schedule K-1, and the S Corp itself pays no federal income tax. The two forms are for fundamentally different tax structures.