Scotch whisky is by far and away the largest selling and most renowned. To be called Scotch whisky, it has to be matured in Scotland for a minimum of three years. However, local spirits distilled in Ireland, Japan, Canada, America and India are also known as whiskey. Scotch whisky is always spelt without an 'e', be it single malt whisky or blended Scotch whisky. On occasion, particularly in the United States, blended Scotch whiskies will be shortened and asked for as simply Scotch.
Japanese whisky, Canadian whisky and Indian whisky are also spelt without an 'e'. It is believed that Japanese whisky is spelt this way as a result of Japan's first whisky distillers learning their trade in Scotland, in the early 1920s, thereafter adopting the Scottish convention. In a similar vein, Canada and India are thought to have embraced the spelling during the time they were part of the British Empire.
Historically, Irish whiskey distillers inserted an 'e' to their spelling to differentiate their product from the Scottish variety. American whiskeys, both bourbon and rye, have in general taken-up the insertion of an 'e'. As you would expect in such a vast country as the United States, which boasts widespread Scottish ancestry, there are some distillers who prefer to adopt the Scottish practice.
The origins of the word whisky, however, can be traced back to the Scots Gaelic word uisge beatha, the 'water of life', which was been Anglicised over time to 'whiskybae' before finally being shorten to whisky. Uisge beatha (or usquebaugh in the Scots English spelling) itself is believed to be an ancient Celtic translation of the Latin acqua vitae (water of life).