The accuracy of a GPS receiver, or how precisely it can measure your actual location on the Earth, depends on the manufacturer's design (more expensive GPS receivers are generally more accurate than less expensive receivers), how you use the GPS receiver, and conditions at the time you are using it. GPS accuracy can range from 10 meters or worse to within centimeters of your true location on the Earth. Besides the design of the GPS receiver, some of the other things that can affect accuracy include atmospheric conditions (a lot of atmospheric moisture can reduce accuracy), the positions of the GPS satellites, and your surroundings (you generally will get a more accurate location if you are using your GPS in an open field versus using it next to a tall building or under dense forest canopy).
A technique that can greatly increase GPS accuracy is the use of differential corrections; however, this capability is not found in most low-cost receivers. See FAQ 29980 "What are GPS differential corrections?"
