A lot of people are confused about what "whiskey" is and what "whisky" is...and for good reason. Why would you have two names for the same thing?
The short answer is that there is no good answer. That's the long answer too. But basically, here's the breakdown:
"whiskey" refers to American and Irish products
"whisky" refers to products from anywhere else (e.g. Scotland, Japan, New Zealand, etc.)
There are a few American distillers that use "whisky" instead of "whiskey" (such as Maker's Mark) but they are rare and should be scolded for going against the trend.
Another point that confuses people is the use of the word "whisky" versus the use of the word "scotch." First, let me point out that I've seen it both as "scotch" and "Scotch" and I've come to the conclusion that the best usage is a capital S when followed by the word "whisky" and a lowercase s when only using the word alone.
As for the actual difference, scotch is a type of whisky - it's whisky that is produced in Scotland, matured in oak casks for at least three years, and contains malt whisky (i.e. whisky distilled from a fermented mash of malted barley). There are a few other legal distinctions that you can read about here, but those are the main three.
And, finally, folks are often confused about what the term "single malt" means. Break this down into two parts - first, the word "single" refers to a single distillery. It is a product from only one distillery, not blended with whiskies made at any other distilleries. Second, "malt" means it is produced completely from malted barley (not, for example, corn, rye, or wheat). So the two words don't really have any relation to each other, but they just narrow the scope of what is in the bottle.