A patent is a property right granted to an inventor by the country in which they live/work. In the United States, it is "the right to exclude others from making, using, offering for sale, or selling" the invention in the United States for a limited period of time (usually 20 years.)
Patents are issued to encourage new inventions by (1) protecting the right of the inventor to profit from the invention, and (2) making public the details of the invention to allow others to build on that knowledge.
The patent literature is a very useful source of business information (Who is developing products in a field? What are my competitors doing?); career information (What companies or universities might want someone with my interests/skills?) and technical information (What technology is available for my purpose? How is a certain type of chemical synthesized?)
One note about Google Patents: It only covers the most cited patents. You sometimes need a more efficient way to search for every patent. Despite this, I do highly recommend it as a general tool.
Example for searching for a impeller for a centrifugal pump, using any of the Primary Search Tools:
If you don't like your results, try using a USPTO Classification Code to look up the exact controlled search term. For example if you are working with a washing machine, you may need to try "agitator" instead of "impeller".