GPS (Global Positioning System) uses radio waves from satellites to very precisely calculate the location of a point on the face of the earth. There are three broad categories of GPS devices: navigation grade, mapping grade, and survey grade. Navigation grade GPS is intended to get you where you're going. The handheld GPS widely available at low cost and the GPS systems in many cars are examples. They're reliable to within maybe 30 feet or so. Mapping grade GPS is somewhat more precise and is used to catalog and map streets, utilities, zoning lines, etc.
Survey-grade GPS is much more complex, and thus more costly. It is extremely precise -- in optimum conditions it's as good as more familiar precision-surveying devices such as total stations and EDM, meaning tolerances of nearly 0.01 foot can be achieved. Good results can often be achieved under adverse conditions, where buildings, trees, and other objects can interfere with or distort satellite signals. Typically under average woodlot canopy we expect "sub-meter" tolerances (i.e. +/- about 3 feet). Actual results are often much better than that. Often we use survey-grade GPS techniques to survey the more remote portions of large rural lots while using more traditional methods on the road frontage and more developed areas. GPS eliminates or reduces the time-consuming and sometimes dangerous tedium of cutting out long lines of sight. It also allows us to much more efficiently gather off-site data that may be relevant (town line monument locations, for example).