With the sakura in bloom -- in some places, anyway -- this is one of the best times for experiencing Japan's wonderful knack for tying just about everything in to the seasons. Clothing, food, drink, design; all seem to resonate with the sakura this time of the year. The sake world's seasonal equivalent of blooming cherry blossom buds is nama-zake, and now is when it is most easily and widely available.
In short, nama-zake is unpasteurized sake. Almost all sake (anything not labeled nama) has been pasteurized twice; once just after brewing and once again after a maturation period or before shipping. This is done by either running the sake through a pipe submerged in hot water (about 65 degrees is the norm), or submerging already bottled sake in the same.
Pasteurization is done to deactivate heat-sensitive enzymes left over from the koji, thus ensuring they will not kick in at higher temperatures and send the sake flavors out of kilter.