Whetstones are the sharpening tools you would use for the best manual sharpening. This is one of the most traditional way to sharpen your knives. You would usually use those after using a steel, though some will only use the whetstone. These knife sharpening stones will really be helpful when you need a sharp edge.
There are few different of types of whetstones, and it is easier to picture those by their roughness. A whetstone can use be rough grit, medium grit or fine grit. The rougher, the more metal it will remove from your blade, the finer and you will use it for polishing your edge.
This also means that you will not sharpen your knives with rough grit everyday. Rough grit are for the big sharpening you’d have to proceed with when your knives really become dull. For everyday sharpening, if needed, you will prefer the fine grit.
Whetstones commonly comes in a block style, and can have two sides, each of a different roughness (rough/medium or medium/fine, medium/super fine, etc).
To keep your stone longer, it s recommended to use a bit of water on the surface of the stone, though some people use oil too. This will also keep too much roughness to scratch your blade. Remember, when you sharpen a knife, you are removing some metal from your blades, and then re-polish it. Adding water will lubricate the stone and facilitate cleaning.
Global have a very nice choice of whetstones, here are few ones you can use for your knives and that you will keep for a long time. Rough grit whetstones can last a lifetime as you will use these less often than others.
On the left is a Global Whetstone (Rough 240 & Medium 1000) and on the right you can see a Global Ceramic Whetstone (Super fine grit, 5000)
, from Metro Kitchen.