When whisky comes fresh off the still, it's clear. But don't mistake it for a vodka or a gin, as the distillation processes are night and day. With whisky, the aim is to capture as much of the flavour of the grain as possible, whereas for neutral spirit (base for vodka + gin), you want 'pure' alcohol; the more separation between the alcohol molecules and the grain molecules, the better the neutral spirit.
When you've got a batch of fermented grain (called a "wash", about 7% alc./vol.), and you heat it, the alcohol starts boiling off around 78°C - but it's not a clean separation, the alcohol that vaporizes off the wash is bringing a lot of the grain with it, called "congeners" -some good, some not good. The copper in the column reacts out some of the unpleasant congeners (sulfites) as the alcohol vapour floats its way up through the still. When it re-condenses into a liquid distillate at the top of the still, the first distillate to trickle out in a whisky run will have an ABV in the high 80s. The ABV will slowly lower over the ten-hour long distillation run. This "run" (slow trickle of whisky off the still) has three phases: heads, hearts, and tails. We only want to bottle the hearts phase (starting around 80% alc./vol.) - segregating out the hearts distillate is the central aspect of the distiller's art. This is what whisky is all about. To make a neutral spirit, conversely, you distill and re-distill until you can condense a distillate that comes out at least 95% alc./vol. You want to lose all traces of the grain. The opposite of whisky making.
If you know you're going to be barreling your whisky for a long time, you can choose to increase your output (and profit) by making a wider and deeper hearts cut - i.e., allocating a greater percentage of the ten-hour run into the "hearts" pot because the oak will help relieve some of the unpleasant congeners you collected from the wide hearts cut. While simultaneously absorbing some bad congeners, the oak also introduces its own flavours into the distillate (vanilla, oakiness). But inside a ten-hour distillate run is some extremely flavourful whisky that is a pure expression of the grain it was distilled from, and with careful, narrow cuts, a tremendous sipping whisky. Unaged whisky is a must for whisky enthusiasts who want to understand what various grains bring to the drink on their own merits, and to explore what effects a grain's terroir has on a whisky.