In the theory of plate tectonics, the earth's crust is broken into plates that move around relative to each other. As a result of this movement, three types of plate boundaries are formed: divergent, convergent, and transform boundaries.
Graphics card temperatures typically range from 30c to 40c at idle and from 60c to 85c under load. Most high end video cards typically have a maximum temperature between 95c 105C, at which point the system will shut off to prevent damage. A video card is not considered to be overheating until it exceeds 90c under load. At this point, artifacting may start to occur up to the point of thermal shutdown.
Under normal usage, most people shouldn't come even close to these temperatures, unless your computer is very dirty or in a very hot room. If you're overclocking your processor or graphics card, though, you're much more likely to see these high temperatures, so knowing these values is important. I usually like to stay at least 10 degrees below them to be conservative. If your computer's running too hot, try cleaning out some of the dust or improving your cooling.