Kvas (квас) is a soft drink made from fermented dark or light rye bread. It is wildly popular not only in Russia, but all across Eastern Europe, where it has been a traditional drink for over a thousand years. Real kvas is brewed like beer and has a very mild alcohol content (usually around 1% or so - hardly enough to mention) due to the fermentation process. Most of the kvas available in the US, however, is made more like modern commercial soft drinks using carbonated water, sugar, and malt extracts for flavor. This method more or less duplicates the flavor but serves to eliminate the alcohol content completely.
There is a huge variation in the flavors of kvas depending upon the brand and the method used to make it. Some are sweeter and some dry, some extremely "beery," some that taste like carbonated rye bread. Some are sweet, mild, and have a flavor that I can only describe as "chocolaty." The best way to find one that you like is to find a store that carries a variety of them and buy several different small bottles to taste, but be warned: if you think that Moxie is an acquired taste, you may find kvas really difficult to enjoy.