There is quite a variety on the market, but all have a V -shaped notch with carbide blades in the bottom of the notch. The knife is dropped into the notch and pulled through. Although simple, the cutting edge produced is not very durable, nor very good. Avoid power sharpeners like the plague . The worst thing that can happen to a knife when sharpened by hand is it will lose its edge.
Motor driven wheels and grinders can ruin a knife.
The original grinding lines are changed, and the blade can be overheated and loses its temper. Professional grinders use motor driven wheels, but it takes years of practice to get consistently good results. Most advice concerning knife care seems obvious. However, there is a lot of abuse going on in today's kitchens. People see in commercials how kitchen knives are abused as hatchets, pry bars, screwdrivers, hacksaws, etc.