From wikipedia (since they explain it better than I can):
Bison Grass Vodka, is a dry, herb-flavoured vodka that is distilled from rye and bottled at 40% alcohol by volume (80 US proof). Its flavour is unique and is described as having woodruff, vanilla, coconut, cinnamon and almond notes.
The rye distillate is flavoured with a tincture of bison grass (Hierochloe odorata), which also gives the spirit its yellowish color. This grass grows in the Białowieża Forest and elsewhere. A blade of bison grass is traditionally placed in each bottle of Żubrówka, though this is largely decorative.
Cracked the Basil Hayden Dark Rye just the now. First sip is harsh, as American often is, but it settled nicely. Not sure it's materially better than the very similar Alberta Rye "Dark Batch", which is quite a bit cheaper, but the BH is not a bad dram by any definition. Spicy rye, smooth canadian rye, sweet port, as it says on the label. Flavors blend nicely. Sipping neat, no ice, and enjoying it, always a good sign for an American product IMO.