Due to the dirth of information here, I'll start.
First off, from the book
Sushi Hygiene and Basic Techniques, from the Organization to Promote Japanese Restaurants Abroad, p63.
Now the term kongosa is a relatively new one for me, but it makes sense. Think of a DMT XXC- you can't call that a simple coarse stone. I'd advise the novice to forget this ultra-aggressive level of sharpening, as it's mainly for repairing severely chipped or damaged knives. Though many categories of grit overlap, I would think it's safe to say that these stones are in the sub-200 grit range.
The arato, or coarse stone, is another stone the newbies are warned to avoid, but eventually you'll realize that it's a critical step in giving a good, lasting edge. These are roughly in the 220-600 grit range, although you can find a wide variety in the cutting speed and level of polish that different examples will produce. These stones are great for quickly establishing and flattening a bevel, and everyone should have at least one. Examples would be the DMT C and XC or Atoma 400 diamond plates (diamond plates double as fantastic fixing stones), Shapton 220-500, and other coarse stones from Naniwa, Beston, and others. Once you have mastered basic technique and angles, the arato is indspensable for good results, as well as your sanity.
The nakato stage, in the 800-1500 range, is where the edge starts to become useful. If you only have one stone, a 1K should be it. With enough work and patience it can address most neglected blades, albeit slowly, and it can leave an edge that if not highly polished is certainly serviceable. Excellent examples are available from King, Naniwa, Shapton, Bester, Suehiro, and almost any producer of note.
Of course, we're all nuts here, so no one would ever be satisfied with a 1K edge. Increasing levels of polish can be obtained with shiageto stones, whose fine grit may range from 2K to 12K or more. Here we tend to focus more on personal preferences in terms of finish and feel, and bring money- these are the most expensive stones.
The preceding has just barely scratched the surface of what's available and what's involved. Hopefully, others will chime in, add to this, or even complain. Fighting over stones and techniques is half the fun.