If you are new to whisk(e)y, haven't drank that much of it and haven't yet invested in an idiot's guide to Scotch then the whole thing can be a little confusing.
To begin with, all three of these terms refer to the same drink.
The basic difference is regionality, and the country, or countries the whisky is produced in.
Whiskey (plural whiskeys), with an 'e', mainly refers to whisky produced in the US (Canada uses the Scottish spelling), such as Bourbon and Rye, and Ireland, such as brands like Jamieson's and Bushmills.
Whisky (plural whiskies), without the 'e', refers to whisky made in Scotland, and other countries who produce whisky in a similar style to Scotland, such as Japan and France.
Scotch however, refers only to whisky produced in Scotland, and is a term most popularly used in film and TV, particularly in America, to differentiate it from Bourbon or American whiskey.
The word whisky is widely regarded to be an anglicisation of the Gaelic word uisge (or uisce) meaning "water", which in itself was taken from the Latin, aqua vitae, used to describe distilled alcohol.
This was later shortened to terms like usquebaugh, and later to oosky, the more it was used in common language.
This later evolved into the word we know and love today.
So why the different spelling between Irish and Scottish whisky?
The definitive reason has been lost over time and where experts have failed to adequately explain it, popular folklore and theory have tried to fill in the gaps.
From the rational (and most likely) theory: That it's simply down to the different regions' spelling of the word, which grew organically over time.
To the outrageously complex and outlandish (and our favourite): That Scotch whisky which, being double distilled, had more fatty acids than Irish whiskey, which being triple distilled, was less likely to go naturally cloudy when ice was added.
In the period before chill-filtration, most Scotch whiskies would take on a turbid appearance when ice was introduced, and despite this being a natural process, many American importers during the late 1800s and early 1900s (wrongly) believed the product to be inferior or flawed.
So they stamped Irish whiskey with an 'E' for excellence, which was eventually incorporated into the spelling of Irish, and by extension American whiskey.
Other versions also say that Scotch in those days, simply wasn't as good as Irish whiskey but we aren't sure that could possibly be true.
This story does not adequately explain why producers in Ireland also spelled it that way, but is a fun tale none the less.
Famously, The New York Times spelled whisk(e)y the American way for all styles of the drink, no matter its country of origin, however this soon drew the ire of Scotch fans who demanded they change their style guide to include the 'whisky' spelling for any whisky not made in Ireland and the US.
So, we hope this helps to explain a little of the differences in terms, and give reason as to why many a Scot will go out of their way (usually in as friendly a manner as possible) to correct your spelling should you get it wrong.
It's also important to remember that whisk(e)y is whisk(e)y and should be enjoyed on its own merit, no matter its spelling, region or style.