A United States (U.S.) patent has three parts.
There are three types of United States patents: utility, design, and plant. The vast majority of patents are utility patents, which when granted are good for 20 years.
U.S. “patent documents” include both patents (PatFT database) and patent applications (AppFT database). The International Patent Cooperation Treaty requires that both be searched for prior art.
Patents issued prior to the Internet (and the computer) must be searched in the official patent office sites using the CPC subject classification system; since they are not born-digital they have not been keyword-indexed.
The USPTO recommends starting your search on their site search box, by combining a few words describing your invention with the phrase "CPC scheme," which should bring up the relevant areas of the classification system (see Patent classification page for further information on the CPC system).
These are the most well-known free sites.