The C corporation is the classic legal entity of the vast majority of successful companies in the United States. The S corporation is used for family companies and smaller ownership groups.
The clearest distinction from C is that the S corporation’s profits or losses go straight through to the S corporation’s owners, without being taxed separately first. In practical terms, this means that the owners of the corporation can take their profits home without first paying the corporation’s separate tax on profits, so those profits are taxed once for the S owner, and twice for the C owner. In practical terms, the C corporation doesn’t send its profits home to its owners as much as the S corporation does, because it usually has different goals and objectives. It often wants to grow and go public, or it already is public.
In most states an S corporation is owned by a limited number (25 is a common maximum) of private owners, and corporations can’t hold stock in S corporations, just individuals. Corporations can switch from C to S and back again, but not often. The IRS has strict rules for when and how those switches are made. You’ll almost always want to have your CPA and in some cases your attorney guide you through the legal requirements for switching.