A patent is a legal document, which is often the first (or only) publication of information on a new area of research. It is a government-granted right to exclude others from exploiting your invention, for a specific time period.
A patent includes a description of the nature of the invention, and is often the first place that information on a new area of research is made public. As patents must disclose (in some detail) the nature of the invention, details disclosed in an existing patent may save you from undertaking unnecessary research.
In most countries patents are issued by a single authority, and are available on the official website. Some useful search tools for finding patents include: