What sets the corporation apart from all other types of businesses is that a corporation is an independent legal entity, separate from the people who own, control, and manage it. In other words, corporation and tax laws view the corporation as a legal "person" that can enter into contracts, incur debts, and pay taxes apart from its owners. Other important characteristics also result from the corporation's separate existence: A corporation does not dissolve when its owners (shareholders) change or die, and the owners of a corporation have limited liability -- that is, they are not personally responsible for the corporation's debts.
To learn more about the corporate structure, read Nolo's article Corporation Basics.
If you need legal advice for your business, talk to a lawyer.