In the United States, a patent is an intellectual property right granted by the Government of the United States of America to an inventor "to exclude others from making, using, offering for sale, or selling the invention throughout the United States or importing the invention into the United States" for a limited time in exchange for public disclosure of the invention when the patent is granted. However, please note that this does not actually confer the right to actually practice the invention by the Inventor, though in most cases, absent some government prohibition on practicing the invention in question (e.g., national security concerns or other regulatory constraints, such as in the pharmaceuticals industry), the effect is that a patent is in fact a legal monopoly to practice the invention in question for a limited period without competition.
This period of exclusivity is the "carrot" that the Founders of the United States wrote into the U.S. Constitution (Article I, Section 8, Clause 8, concerning patents and copyrights) in order to promote the progress of the sciences and useful arts, and help ensure that the engine of commerce gains power. In other words, the Founders wisely recognized that incentives matter to individuals and to businesses, both small and large.
There are three basic types of patents: Utility patents, Design patents (called "Industrial Designs" in many foreign nations), and Plant patents, each of which is briefly discussed later on this page.