A knife sharpener is a tool which is used to sharpen the blades of knives by grinding them down to even out the surface of the blade. A wide range of knife sharpeners are available from kitchen supply stores, ranging from commercial electric grinders to simple whetstones. This tool is extremely useful to have around, as a sharp knife cuts much more effectively, and regular sharpening can extend the lives of your blades. Many communities also support a professional knife sharpener who will sharpen blades for a small fee; sometimes a professional can handle special blades like scissors in addition to standard kitchen knives.
When a knife is made, it is ground into a very specific shape which is designed for a particular function. Numerous styles of grind can be found on the market, ranging from serrated blades which are ideal for bread and tomatoes to convex ground blades designed for cleaving dense foods like melons. After grinding, the blade is sharpened so that it has an edge for cutting. Over time, this edge will break down, even when a knife is carefully handled, and a knife sharpener is used to refresh this edge.
To use a hand held knife sharpener like a whetstone or a sharpening rod, the knife is run along the sharpener in smooth, even motions to clean up the blade. These sharpeners are made from coarse materials which smooth out irregularities in the blade. Other sharpeners have an electric component which rotates or jiggles a grinding surface while the user holds the blade against it. Electric sharpeners like grinding wheels have to be used with care, since it is possible to oversharpen a blade.
Knife sharpening is not impossible, but it does require some skill. It is important to angle the blade as you sharpen, and to maintain the angle to ensure that the blade is being sharpened, rather than simply ground away. As you sharpen, a burr will appear. A burr is a thin lip of metal which overlaps onto the unsharpened side of the blade, and it is a good sign; once a burr appears, you can switch sides to refine the edge and get the knife's edge thin and crisp for best cutting.
A number of tricks and tips for using a knife sharpener can be found on websites which cater to chefs; most explanations wax on for several pages since they include discussions of different knife types and materials as well. What you need to remember is that water or oil are not necessary for the sharpening process; you can use a knife sharpener dry, and this is in fact better for your blades. Also be aware that regular sharpening keeps your knives healthy, along with other basic rules of knife care like never running knives through the dishwasher and using knife guards to protect the blades.