If you're a homeowner, this section of the website can link you to some good general information on what well tanks, heating tanks and thermal expansion tanks do and how they work. There's also a link to consumer literature and to the Water System Council, a non-profit organization that works to protect our groundwater resources.
But please don't get the wrong idea. We're not suggesting you go out and try installing this equipment yourself. A well tank is not like a toaster–you don't just plug it in. You're dealing with a sophisticated piece of equipment and compressed air must always be treated with respect.
Working with pressure tanks is a job for water systems and plumbing and heating professionals. They can find the system that will work best for your house and make sure it's installed safely and correctly so it will provide many years of service.