Knife sharpeners come in a wide variety of styles and require a range of skills to use. A good quality knife has normally cost good money and so should command a good quality sharpener.
Steel: This is the favourite of chefs and consists of a round or oval bar with a handle and small guard. The bar is very hard and is ridged or sometimes covered with industrial diamond dust. They come in a variety of lengths and should be long enough to accomodate your longest knife. To use hold in the left hand (for right handed people) horizontally at chest height and stroke the knife along the top and then the bottom at an angle of 22 degrees ensuring that the entire length of the blade edge slides along the steel. Speed is not necessary and pressure should be gentle. Practice is necessary to achieve a good edge and this should be checked every 4 or 5 strokes. An alternative sharpening style is to hold the steel in your left hand as though you were going to stab with it. Place the tip on the work surface and stroke the knife in a circular motion on one side and then the other side at an angle of 22 degrees. The sharpness can be checked by drawing the knife across the edge of a sheet of paper which the knife (if sharp) should cut with ease. NB. The diamond covered steel can be a good deal more aggressive than ordinary steels and should be used with care.
Stone: The best stone for knives is the hard Arkansas stone. This is a natural stone and the worlds finest finishing stone being the least aggressive of all the natural stones removing the smallest amount of metal to create a razor sharp edge. The angle of 22 degrees still needs to be maintained but on a stone this can be aided with the use of a guide. This clips onto the back of the knife to be sharpened maintaining the blade at the correct angle to the stone. Vegetable oil in very small quantity can be used to lubricate the stone and slow movement and only gentle pressure applied.
Ceramic wheel: This is an enclosed pair of bevelled wheels approximately 3/4 inch in diameter. These are normally surrounded with water to remove minute shards of metal. The knife is placed between the wheels, gentle pressure applied and the knife drawn towards you through the wheels. This can be quite an aggressive sharpener and does not work as well with serrated knives.
V sharpener: These come in a variety of different manufacturer names and consist of one and sometimes a pair of v shaped metal slots through which the knife blade is drawn. The v basically scrapes the metal from the knife. These need only very light strokes and have a limited life and can be extremely aggressive. Their one plus point is that they are the cheapest of sharpeners but I would not recommend them for quality knives.
Ozitech: This is actually a brand name which sets it aside from the others. It has though proved to be very successful. It comes in a plastic shell of two halves that opens to present a series of diamond covered sprung fingers. The knife blade is drawn through the fingers that are set to the correct angle these give to pressure and so regardless of how hard you press the angle remains constant.