Global Positing Satellite system (GPS)
This is a huge subject, and many pages cover it well. Simply put, for about $100 or up to whatever you want to pay, you can buy a small, usually hand-held, device that uses a system of satellites to tell you exactly where you are, with an accuracy of less than 20 feet in most cases. These are a few links to helpful information, if you would like to know more. Some of these are commercial sites, but they have useful information in the midst of their sales material.