Five crucial elements are involved in brewing sake — water, rice, technical skill, yeast, and land/weather. More than anything else, sake is a result of a brewing process that uses rice and lots of water. In fact, water comprises as much as 80% of the final product, so fine water and fine rice are natural prerequisites if one hopes to brew great sake. But beyond that, the technical skill needed to pull this all off lies with the toji (head brewers), the type of yeast they use, and the limitations entailed by local land and weather conditions.
Sake brewing looks something like this. Rice is washed and steam-cooked. This is then mixed with yeast and koji (rice cultivated with a mold known technically as aspergillus oryzae). The whole mix is then allowed to ferment, with more rice, koji, and water added in three batches over four days. This fermentation, which occurs in a large tank, is called shikomi. The quality of the rice, the degree to which the koji mold has propagated, temperature variations, and other factors are different for each shikomi.
This mash is allowed to sit from 18 to 32 days, after which it is pressed, filtered and blended.