An S corporation is a C corporation that has filed an election to be taxed as a “pass through” entity. This means that instead of being taxed on its income, it passes the income through to the shareholders, who are then taxed on it. An S corporation files Form 1120S (which is a tax return different from that used by a C corporation). This election is made on Form 2553 and must be filed no later than 60 days after the beginning of the year in which the election is to be effective. For instance, an election for 2008 must have been filed by the 60th day of 2009 (March 1). Once the election is made, it is effective until revoked.