Russian Kvas is really a strange drink, at least from a categorization standpoint. Looking at the bottle, it looks like any other soda or soft drink (aside from the Russian writing). It is carbonated, but that's because it's fermented like beer.
Translated from the Cyrillic квас, known in English as kvass, kvas, or sometimes bread drink, kvas is made of rye bread and fermented to make a slightly alcoholic beverage. The alcohol content is so low that it's considered safe for children to drink.
I tried Nikola Kvas for the first time at my Russian friend's house. He described the brown drink as "beer soda." Obviously I had to try it. While it does have a beer-ish taste, it's tangy and tastes a bit like hard apple cider. It's definitely not sweet like most soft drinks, and took a couple sips to get used to. Kvas is not really the greatest drink for refreshment, since it's a little heavy. It's great for drinking with a hearty meal though like meat and potatoes or just for the taste.
Nikola is a Russian kvass company, whos name sounds like "not-cola" in Russian. It's one of Russia's national drinks, and there has been a great revival of Kvas drinking there, as a push back against the big foreign soda makers. The Russian kvas recipe is traditionally made from fermented wheat, rye, or barley with fruit sometimes added for flavor.
I took a look at the Nikola kvas drink ingredients, and it seemed to be a blend of all three grains, using apple as a base. That would explain the hard cider taste. This is clearly a commercial production though, made in similar ways to other soft drinks. I'm curious to try a home-made kvas to see how they differ in taste.