A patent is a government grant that gives you, the inventor, the right to keep others from making, using or selling your inventions. You could think of a patent as a contract between you and the government that protects your rights as an inventor. In return for this protection you give the government a complete description of your invention.
Patents are vital resources for businesses, researchers, inventors, academics, and others who need to keep abreast of developments in their fields.
Patents are also an important means of sharing know-how, because each patent document describes a new aspect of a technology in clear and specific terms and is available for anyone to consult.
Espacenet offers free access to more than 70 million patent documents worldwide (70+ countries including Canada and the U.S.), containing information about inventions and technical developments from 1836 to today, updated weekly. Often considered the best place to start a patent search.
See this video for an introduction to searching in Espacenet
The USPTO houses full text for patents issued in the U.S. from 1976 to the present and TIFF images for all patents from 1790 to the present. (You can install a free TIFF reader that is designed in part for reading U.S. patent office files).