A Global Positioning System: the name speaks for itself. At any given, time your position on the earth can be plotted and remembered by a GPS. The GPS uses satellites placed in what is called a geosynchronous orbit. Anyway, it means the satellites stay in exactly the same positions at all times. Since they do this, your GPS can locate its position at any given time. GPS can have its problems. The first problem is accuracy. They are capable of locating precisely the same spot, but they can also be off a good bit. Even the instruction book begins by warning about the accuracy of the units. I have not had the experience I should, to criticize or compliment these products. I have been exposed to several Lowrance units and have had good results. The way I tested a particular Lowrance unit was as follows: First I dropped a buoy. Then I stopped the boat right next to it and recorded my waypoint (position). I then drove about a mile away and then headed back. First the arrival alarm sounded and I could barely see my buoy. I then thought well it is very inaccurate. Then I realizes that I needed to go by the position numbers (example 45,345 West and 12,867 North) Then my accuracy increase to a few feet of my buoy. My best advice is try to find one you can borrow and just try it before you buy it. On the other hand, if you fish many different lakes, wide open waters, or hunt, you may want to try it very soon because it does work surprisingly well. Like computers these units get better and better with each new model and the are getting cheaper too.