Sake is what comes hot in a small cup when you order sushi, right? Most of the time that's what you get, but there are cold sakes and cloudy, unfiltered ones. There are all sorts of levels and grades of sake: some that are rare, small batch, and unique--and some not.
What is Sake?
Sake is brewed like beer. Raw starchy ingredients in beer (hops) get broken down by molds into simpler carbs that yeast can then convert into alcohol. In sake, the raw starchy material is rice but the rest of the process follows suit. Sake rice, like sushi rice, is its own special species. Sake rice is generally larger grained because the outside layers get stripped away to produce a refined, purer starch center. The amount of "polishing" of the rice determines some of the grade and quality of the sake. For more about sake grades and how it's made, check out an upcoming piece I wrote for Wine Enthusiast Online about sake.
Why Warm, Anyway?
Warm sake is a welcome respite on a cold day, but the raised temperature also evaporates the alcohol more aggressively, filling your nose and dampening other smells. For this reason, often cheaper sake is heated to add a bit of panache as well as mask its weaker character, which is why that's what you usually get at average sushi bars.
Sake vs. Wine
Is there any point in comparing? Some would say yes, others no. In the end, they're very different beverages with strikingly different methods and techniques employed, different histories, and very different characters. In my humble opinion, drink what you like in the context that gives you the most pleasure. Just like wine or beer, I can easily be tempted to try sake I've never heard of.
However, there are a few striking differences worth noting. The flavor of sake comes primarily from the microbes used to convert the raw rice starch into simple sugars and then alcohol. Sake brewers spend a great deal of effort tending their microbes to ensure the right ones with the most desirable flavor characteristics get into the mash. And then there's brewing technique, an entire science unto itself.
Wine grapes, unlike polished rice nuggets, have natural simple sugars in abundance, so there's no need to use molds to convert the complex carbs first. Accordingly, the flavor of wine comes in large measure from the grape juice and skins, along with the methods used to tease the wine out, not mainly from the microbes. In making sake, the microbes and the technique are much more pronounced in the flavor of the final product.
Grapes provide a range of powerful flavor profiles--from Sauvignon Blanc to Syrah--while sake rice provides a more subtle, narrower set of foundation flavors. Sake might be most comparable to Chardonnay in the fact that this grape, by contrast to almost any other varietal, shows the least character on its own but get enhanced and molded by the maker's choices and technique most assertively.
Range of Sake
Sake may start with a narrower palette of raw flavor profiles, but that doesn't stop the innovation. Brewers across Japan use different techniques and timing to achieve dramatically different results. Unfiltered nigori sake, for example, has rice particles suspended in the final product, lending a sweeter, more robust flavor and more textured mouth feel. Daiginjo, by contrast (a highly refined grade of sake), uses almost perfectly neutral rice starch and lots of filtering to produce a very subtle final product.
If you're looking for a place to start exploring sake, take yourself and one of your best dinner companions out to a good sushi bar and order a junmai to go with your meal and nigori for after. After that, you're on your own.