What it is:
A sharpener. For knives.
Why you should have one:
Pro chefs are adamant about keeping a razor edge on their tools, and sharpen their knives before every use. You should do the same.
How to use it:
That all depends on which one you choose. The sharpener on the top left, from
Vulkanus, is made of heavyweight Austrian-made steel and uses sharpening bars in a V formation, with spring tension, to deliver the perfect amount of friction needed to sharpen your blade, without scratching it. It's also designed to take both straight and serrated blades, unusual for a sharpener. All you need to do is slide the knife through the V from the handle to the tip, with a slight downward angle. The levers automatically adjust to the original angle of the blade.
The sharpener on the bottom right is
Kyocera's new steel knife sharpener. It's got an advanced ceramic sharpening wheel, (close in hardness to diamond), that puts an incredibly sharp edge on knives. You can only use this one on steel double-edged knives though. And it won't work on serrated blades either. But it does make it easy to get a serious edge. Hold onto the ergonomic handle and slide the blade back and forth several times. Then go slice and dice yourself silly.
The sharpener on the right is what's commonly called a Chef's Steel. The one shown, also
from Kyocera, is made of ceramic. A little more challenging to use for the average guy (and a little more likely to result in a trip to the ER to reattach a thumb), the steel is used to remove imperfections from the blade, to keep it cutting smoothly between major sharpenings.
What to look for:
First decide which sharpener suits your knives. Then look for quality materials. Heavy steel. Comfortable handles. Ease of use, without doing more harm than good. The ones pictured are available from many online retailers, and to give you an example of the price difference, the Vulkanus sharpener will run around $100, while the Kyocera ceramic, and others in that style, run under $20. Which isn't a bad investment to keep your tools in top shape.