Otto,
I am quite a rookie when it comes to Sake, and I tend to drink it only occasionally. However, during those times, I find that I enjoy the premium Sake grades the most. That is why I look forward to these Sake tastings at our local wine shop. One of the owners used to travel frequently to Japan for his business and learned quite a bit about this subject.
I found a pretty interesting web site with information about Sake. Give this one a try and see if it answers your questions:
http://www.sake-world.com/index.html.
In a nutshell (or rice shell, I guess), the Junmai designation means the rice grains have been milled (or polished) to remove at least 30% of their outer layers, which tend to contain contaminents. If you see a bottle labeled Seimaibuai 65%, it means the rice has been milled by 35% and it can be considered Junmai. Junmai also indicates that the resultant beverage has not been fortified with brewer's alcohol.
The Junmai Ginjo designation means the grains have had at least 40% of their layers removed. The Junmai Daiginjo has at leaset 50% removed. The greater amount of milling produces rice material of greater purity, and will produce higher qualiy Sake.
The Nigori designation simply means unfiltered, and the above designations still apply. A Sake labeled Junmai Nigori would indicated an unfiltered Sake that started with rice milled to removed at least 30% of its outer layers and has no fortified alcohol.
The Nihonshudo rating is an indicator of sweetness, but here is where it gets a bit confusing to me. I have seen references to the scale running from -15 to +15, -5 to +15, -6 to +10, and -3 to +12. It might be that the scale itself is very broad, but in reality the beverages produced fall within a more narrow range. In general negative numbers are sweet and positive numbers are dry, with gradations along the way. It really is a measure of specific gravity, and can be influenced by more than disolved sugars, so it is best used as a general indicator.
One final note of interest from our wine shop. Drink Sake when it is young. They only buy it in the spring when the new shipments arrive from Japan.
I hope this information has been useful, and that you can find some very pleasant Sake as a result.
-- Clark