Sharpening is really two processes: Grinding and honing. Grinding is simply the removal of metal. Honing is a precision abrasion process in which a relatively small amount of material is removed from the surface by the means of abrasive stones. Once you have the right shape, usually using a more aggressive grit, you then switch to a finer grit to hone the edge.
Therefore, the first step to choosing a whetstone is to determine your intended purpose and then choose your whetstone accordingly. For instance, when sharpening tools that do not require a fine edge, you should choose a relatively soft, coarse, stone such as a Norton Crystolon water stone. However, for sharpening tools that do require a fine edge, a somewhat harder Norton India oil stone would be a good choice. But, for sharpening hunting knives where an exceptionally fine edge is required, a Novaculite or Coticule oil stone would be the best but, most expensive, choice. So, the process of choosing the correct whetstone for any given purpose is to first determine how fast you would like for the stone to cut and how fine an edge you need, and then choose either a soft, coarse, stone or, a hard, fine, stone of the appropriate type and grit.
Visually, a very sharp knife has an edge that is too small to see with the eye; it may even be hard or impossible to focus in a microscope. The shape near the edge can be highlighted by rotating the knife and watching changes in reflection. Nicks and rolled edges can also be seen, as the rolled edge provides a reflective surface, while a properly straightened edge will be invisible when viewed head-on.
Sharpening on the other hand is reupholstering the furniture or telling the hair stylist to give you a new look. Material is going to be removed from the edge of the blade. There’s no way around it. How much is removed will be a function of just how dull the knife has become or whether you’re sharpening to compensate for a chip in the edge or because the tip has broken off. If your knife is not damaged and you have it sharpened twice a year very little material will be removed each time, yet it may still be enough for you to notice just by looking carefully with the naked eye.
Now you may wonder, if a finer stone produces a better cutting edge why do I need a coarse stone at all? The answer is that finer stones work much more slowly than coarse ones. The amount of metal a fine stone removes with each pass is much less than what a coarse stone removes. If you have an edge with a good shape to it from a coarse stone, this is not a problem because the fine stone doesn’t have to remove much metal to improve the edge. If the edge is dull, it will take many times longer to reshape it with a fine stone than it would with a coarse one.