What? A mixture of whiskies from various grain distilleries. Grain whiskies are distilled in column stills to high strengths, from a beer of largely unmalted cereals (typically maize or wheat).
Where? Currently Scotland, as few countries would have enough grain distilleries to even bother blending them!
When? It seems like John Glaser pretty much made this category up, when he blended the consistently excellent Hedonism Despite there being so few examples, the Scotch Whisky Association included it as one of their five categories of Scotch Whisky in 2009.
How? Hedonism was created out of a love of Grain Whisky's flavour, but Snow Grouse feels like a fresh approach to serve and positioning (they suggest chilling it in the freezer).