I have spent the last 10 days in Japan and tried to come to terms with sake (not the major objective of this trip). I thought I might share some basics with you. The two major dimensions by which sake is distinguished are the amount of rice grain which is used and the brewing method.
Junmai is the basic variety, based on rice, water, yeast and koji (mold). Honjozo has 30% or less of the rice grain milled off. This gives the sake more intensity of flavour. The next level is Ginjo, where up to 40% is shaved off, and the most intense sake is Daiginjo (Dai-ginjoshu), where up to 50% of the rice kernel is taken off.
The previous sakes usually have about 15% alcohol. Then there is Gensyu, which is not diluted with water and has a 17-20% alcohol content. It has deeper and richer flavour. There are some other, less important variations.
Within each category, you can have higher and lower quality. This is also subjective, and depends on at least five dimensions: dry-sweet, acidity, savouriness, flavour notes (apple and banana are often dominant), and fragrance. Most highly regarded sakes are those, which have these dimensions in harmony and have a quick disappearing finish (very different from wine assessment).
I found it quite difficult to assess sake in this brief period of time. It is as complex as our wine, but maybe this note helps as an initial introduction.