The patent is one of three legs of the tripod of intellectual property laws in the US. Where trademarks were designed for logos and whatnot, and the copyright was designed primarily to protect authors, the patent is designed to protect the inventor.
Article I, Section 8 of the Constitution states that: "The Congress shall have power ... to promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries."
Patents were designed with both the inventor and the public in mind. It encourages the inventor to invent, and to share his inventions, by guaranteeing that no one else will be able to imitate his invention without his permission. In exchange for the monopoly granted to the inventor, the inventor discloses enough data about his invention for someone else skilled in relevant area(s) to be able to reproduce and use the invention.
A number of different things can be patented: "any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent".1 You will note that the invention must be both new and useful. The first condition is a rather major one: a patent can be voided by the Commissioner of Patents if someone sends in prior art and requests a reexamination of the patent.2
Patents last a long time. "[The] grant shall be for a term beginning on the date on which the patent issues and ending 20 years from the date on which the application for the patent was filed". So, for 20 years after you file the patent, the invention is yours and yours alone, and you get to decide who uses it and who doesn't. However, you must enforce this yourself: it is not the patent office's job. If someone infringes upon your patent, you sue them.3 You can seek both damages (money), and an injunction (an order by the court not to do it again).
On the whole, patents are (in my opinion) one of the better parts of intellectual property. It seems to be a well thought-out and well-designed plan, and I believe it is serving its purpose quite well.
- US Code, Title 35, Part II, Chapter 10, Sec. 101. (http://www4.law.cornell.edu/uscode/35/101.html)
- The process of submitting prior art and requesting a reexamination is set forth in Chapter 30 (http://www4.law.cornell.edu/uscode/35/pIIIch30.html)
- US Code, Title 35, Part III, Chapter 29, Sec. 281: "A patentee shall have remedy by civil action for infringement of his patent" (http://www4.law.cornell.edu/uscode/35/281.html). Infringement is defined in Chapter 28 (http://www4.law.cornell.edu/uscode/35/pIIIch28.html).
I got a lot of this from the US Patent and Trademark Office's page on patents, at http://www.uspto.gov/web/offices/pac/doc/general/index.html