So malt versus grain are two different ways of making your whiskey. Malt refers to the process by which you germinate barely to make some of the sugars in the actual cereal grain more accessible. When you sprout it, when you germinate it, it's called malting the barley.
You set it, you let it soak, and it starts to form the seed. The little green tail comes out, then you set it out and you dry it. That starts a enzymatic process by which the longer more complex starches get turned into sugars which are more easily digestible to yeast, which will give you the sugar you need to turn into your alcohol.
Grain on the other hand is a more involved process and requires, you know you take mechanical action to really grind it down and make your longer chain starches into more available sugars. Malts generally, it's the same way you make beer.
Even in Scotland when they make the whiskey, they start it and they call it the small beer. It's a non-hops beer basically. Which explains why Scots are so good at making whiskey and so terrible at making beer. On the other hand when you have a grain whiskey, even a bourbon for instance, where you make that out of corn, they take longer to ferment, there's less available sugar, and you get a very strongly different, strongly contrasting end-product.
Barley's tend to be rounder, a lot softer on the tongue, and they'll take on a lot more character from the different processes by which they're made, whereas grains tend to taste more or less like grains. I mean rye tastes like rye, tastes like rye - you can get different expressions, but that sharp spicy quick character that you get from the rye grain is pretty universal.
You can get different expressions, but really it's going to taste more or less like rye. Mostly the same way that bourbon really tastes, it's either soft or sweet or it's aggressive and higher proof with more rye in the batch. So that's a quick run-down of malt versus grain.