Synthetic waterstones are more consistent in sizing and quality than are natural stones, and are more readily available at more moderate cost. Grits range up the scale (numerically) as they get finer, so the 250 grit waterstone is for heavy duty roughing work, while the 6000 grit is a polishing stone, as is the much more expensive 8000 grit. Very coarse stones are used for maximum and speedy removal of metal. Medium stones refine edges and create the burr. With finish stones, you do the final honing and polishing of the edge. Coarse stones are available in 250 to 400 grit. Medium coarse stones are available in 800, 1000, 1200. Some class the 2000 grit as medium coarse, also. Finish stones are available in 4000, 6000 and 8000 grits.
Waterstones also come in shapes for use with turning and carving tools, with cone gouge sets consisting of 180, 1000 and 4000 grits. Water slipstones are available in 1000, 4000 and 8000 grits, each in a four piece set that offers enough varied surfaces to sharpen just about any gouge or V shaped carving tool. Carver's waterstones come in sets, one at 1000 grit and one at 4000 grit. These are packages that contain enough varied faces to sharpen any carving tool--they're generally too small to be useful with turning tools, though some mini turning sets are small enough to fit the shapes.
Sharpening Stone Comparison
Preparation of the Stones[b1]
Waterstones need to be pretty well soaked before use, so 20 to 30 minutes soaking in tepid water is recommended (they may also be stored in a water bath). Before you begin using your stones, take a waterproof marker and print the grit number on both ends and both sides. The grit markings on many stones are right on top, and disappear with the first use. If you use the stones daily, that's probably not a problem. If you use the stones less often, you'll find that you forget which grit is which. The difference between 250 and 2000 grit is easily felt, but the difference between 4000 and 6000 is not. Marking is easy.
Bubbles rise to the surface as stones load up with water. Coarse and medium coarse stones can be left in water without hurting the stones. Remove finish stones from water when the sharpening session is done. Never, under any circumstance, add oil to the waterstones, whether directly or in the water used to soak the stones. To get immediate metal removal, when getting ready to sharpen use a small Nagura stone to prepare the polishing stones for use. Rub the stone in a circular motion over the finer grit stones so that the abrasives and water form a slurry, which is what really does the cutting and polishing for you.
Setting the waterstones into holders makes them easier to use, as the holders are slip resistant, using the back edge of the holders to keep a grip on the bench.
Set Up A Sharpening Progression [b1]
An orderly progression tends to make it easier to maintain sharp tools, regardless of the system you're using to do the sharpening. Once the progression is set up, your initial step in sharpening is to decide where in the progression a particular job needs to be started--not all sharpening jobs require you to remove nicks, for instances.
▪ Wet down the waterstones. About 20 minutes is enough.
▪ Remove any nicks with a coarse or medium coarse stone. The choice of grit depends on how much material needs to be removed from the bevel in order to leave a clean, true edge: use 250, 400, 800 or 1000 grit stones. Use the 250 and 400 grit stones only if a really considerable amount of material has to come off. They leave scratches for later grits to remove. 
▪ Establish a flat bevel with the appropriate grit--for new tools, use a coarser grit, and if there is damage, use a coarser grit. Most start-up sharpening sessions can use 1200, or even 2000, grit stones if there are no nicks. Move the edge back and forth over the stone.
▪ When medium coarse sharpening is done, a burr is created. Move over to a finish stone, and alternately hone back and bevel sides until the burr is removed.
▪ Finishing or polishing can be accomplished in a single stage, using any stone from 4000 to 8000 grit. A better edge results if the tool is first honed on a 4000 grit stone before its final polishing using a 6000 or 8000 grit.
▪ Every so often, check stones for flatness. When stones are no longer flat, you must flatten them. A straightedge held on the top helps. View at eye level to see what kind of gap appears. If there is a gap, flatten the stone's top: use a flat surface (tempered glass works well, but a small sheet of phenolic plastic is also good). Glue down 220 grit sandpaper for most flattening jobs (if the stone is really a mess, you may want to go down to 120 or even 100 grit). Flatten finish stones by rubbing them against each other.
▪ Ceramic flattening plates make the entire stone flattening process fast and simple.