I suppose the most famous region for producing whisky is Scotland. In Scotland the Scotch Whisky Association makes rules for what can be called whisky in Scotland. Scottish whiskies must be aged for at least 3 years in oak casks and the only ingredients allowed are water, grain, and yeast. Even though it is well recognized that a great deal of the flavour of whisky comes from the oak casks, it is not termed in ingredient (which I find strange).
The most commonly sold and consumed Scotch whisky is the blend. A blended whisky is made up of malt whisky and grain whisky. Malt whisky is made from malted barley and grain whisky can be made from any type of grain. Single malt whiskies are malt whiskies that come from a single distillery.
There is some controversy over who produced whisk(e)y first, the Scots or the Irish. Either way, Irish whiskey has just as long a history as Scotch whisky. Notice that I am spelling whisky for Scotch and whiskey for Ireland. That’s the convention which has something to do with Irish distillers trying to differentiate their product from what they believed was inferior Scotch whisky. Instead of malt whisky, a lot of Irish distilleries produce partially malt whisky. Basically, they mix malted barley with unmalted barley to make the beer that is then distilled into spirit. This has something to do with a tax that was put on malted barley back in the old times and the traditon lives on. Some Irish distilleries do produce malt whisky but a majority of them focus on making whisky for blends.