Well to begin with, and in the spirit of being thorough (and not just pedantic!), the use of the word Sake in the western world is, while not entirely incorrect, slightly vague. Sake 酒 means 'alcohol' or 'an alcoholic beverage' in Japanese and could refer to any variety of alcoholic drinks, including Awamori or Sōchū for example.
日本酒 Nihonshu (literally meaning 'Japanese alcohol') is the specific term used in Japanese when talking about what we call Sake. Just in case you're in an Izakaya one night and would like a glass of it, now you know. In order to keep things simple here, we'll just stick with calling it Sake though.
Before we talk about ingredients, perhaps it's time for some myth debunking. Sake is not a spirit. Its alcoholic content is similar to that of a fortified wine, ranging from 14% to 22%, (sometimes much lower), and more often than not around 15% to 17%. It doesn't compare to the strength of whisky or vodka as people often assume.
Abv wise (alcohol by volume) you can think of it as a slightly stronger glass of wine. I do exactly this, and if using a wine glass, just pour a slightly smaller glass than I normally would wine. It is important to note however that Sake is not a wine. Yes, we (in the UK) often call it rice wine but for reasons we're just about to get into, the process by which Sake is made is very different to that of wine and moreover the flavours of the resulting beverage can be very different.
In terms of basic ingredients, Sake is made from rice, water, yeast and koji. The first three ingredients are likely well known, certainly to anyone with an interest in alcoholic drink production. Koji however, probably less so. Koji mould (or in particular 'Aspergillus Oryzae') is the magic ingredient that, allows Sake to be produced. More specifically, the enzymes it produces turn the starch inside the grains of rice into sugar. This is essential as without sugar (glucose), yeast has nothing to feed on and fermentation can't begin.
Unlike the production of wine for example, where sugars are already present in the grapes, or beer and whisky, where the process of malting barley activates the grains' own starch to sugar production, rice without koji produces no sugar, and therefore no sake. Sake is also different to wine and beer in that the koji and the yeast do their work in the tank at the same time. This process is called 'multiple parallel fermentation', and is unique to sake.
On the whole, and aside from specialist styles such as 濁りnigori (roughly filtered sake and therefore cloudy) or 古酒koshu (coloured due to prolonged aging), you can expect a clear almost colourless appearance with perhaps a very slight hint of yellow or green and a similar viscosity to wine.
Sparkling sake is also available and well worth seeking out, and stands as a testament to the ongoing development and innovation of sake brewing in Japan.
Aroma and flavour
Nose and palate characteristics range from floral and fruity, cereal, grain, nuts, herbs and spices to lactic, dairy and more. Refined brewing techniques and the resulting 吟醸 ginjō styles express fruits such as banana, apple, pear, lychee and strawberry etc. A 本醸造 honjōzō style might be more earthy, meaty and peppery.
Sour fruits, apricots, baked apples, mozzarella, honey, caramelised nuts, toasted cereal and coffee flavours are all just some of the flavours you might find drinking sake.
Dry 辛口karakuchi and sweet 甘口 amakuchi styles are both available but, while high acidity shows in some sake, acidity levels are around a fifth of those in wine. Umami levels in sake range from light to high textures range from creamy, rich to crisp and refreshing. Long lingering finishes to short, sharp and clean.
All said, there's a wealth of flavour and aroma to be found in drinking sake and some of the best examples exhibit fantastic complexity and depth.
Of course, no amount words here will do any justice to this fantastic brewed beverage that I'm becoming increasingly fond of. It would be much better to go pour a glass and find out for yourself. In fact I highly recommend doing so and hope you enjoy.