Form 1120
The C corporation's yearly federal income tax return. It reports the profit and the 21% corporate tax.
C corporations file Form 1120, U.S. Corporation Income Tax Return. So does an LLC that elected corporate tax treatment and stopped there, without going on to S status.
An S corp passes its profit through. A C corp pays its own federal tax instead, at a flat 21% of taxable income under Section 11(b). When is it due? The Form 1120 instructions say the 15th day of the 4th month after year end, so April 15 for a calendar year. A company whose year ends June 30 has a month less: the 15th day of the 3rd month.
Expect to owe $500 or more and the company makes its own estimated payments, per Publication 542.
Dividends are where it stings. The company has already paid tax on that profit, and shareholders pay again on their returns, which is the double taxation people talk about. Salary to an owner-employee works differently. Form 1120 deducts it, so it's taxed once, as wages.