Source: Mariners Learning System, By Captain Bob Figular
Six basic functions are common to almost all GPS units although the terminology and the operating procedures required to use them vary from one manufacturer to another.
The most fundamental of all is position, given in terms of latitude and longitude. If you plot your position on a chart it is relatively easy to measure the range and bearing to a waypoint. Your GPS receiver can do this too, except that it uses trigonometry, rather than geometry.
By comparing your vessels current position with its position a few minutes ago, you could work out the direction and distance you have travelled and, from that, work out your speed over the ground. These two are standard functions on your electronic GPS and are usually called CoG (course over ground), and SoG (speed over ground). CMG and SMG for course made good and speed made good are common variations, as are TRK and VEL for track and velocity.
In practice, most GPS receivers use a more sophisticated technique for working out CoG and SoG, but both functions can be adversely affected by positioning errors and become increasingly suspect at low speeds. On a stationary boat, in particular, the CoG display is likely to fluctuate randomly.