Patents provide important technical literature. There are approximately 10 million patents in the U.S. The US Patent and Trademark Office (USPTO) is the official site and answers many questions concerning the patent process. A patent for an invention grants a property right to the inventor, usually for 20 years. The USPTO is not the best place for patent searching, though, instead search Google Patents or Patent Lens, or the sites listed further below. Both patent applications and granted patents are included.
This image (click to enlarge) shows the major parts of a patent. Purdue University points to several good tutorials on patent search strategies, including a discussion of patent classification systems. See also this guide's tab on patent classifications.