Whisky or whiskey is a distilled spirit made from a base of grain, which is typically aged in wooden barrels for some period of time. The most commonly used grains are barley, corn, wheat, and rye. The word itself is derived from a Gaelic term, "Uisge Beatha," meaning "water of life." The spirit has been around for many centuries but today's whiskies most likely do not bear a lot of resemblance to their ancestors. Todays whiskies are aged and take on the color of wooden barrels whereas early whisky was most likely clear and of a significantly different style.
The alternate spellings arise from both marketing and industrial evolution. At one time Scottish "whisky" reigned as king of the market, until along came the invention of the column, or patent still by Stein (later modified by its namesake, Coffey.) Newly column-distilled Irish whisky was re-branded as "whiskey" and touted as superior. Long story short, Scotland's distillers still use "whisky" as their spelling while the Irish use "whiskey." To complicate things further, many of the distillers in the States tend to use whichever spelling their families claim roots to. (Though in the U.S. the legally correct spelling is "whiskey")
Today whisky can be made in either pot stills or column stills and has a few major classifications according to distillate and method of production.
Whisky can range in aroma from very fruity and sweet to very cereally and malty grain. The wood aging can impart a wide range of aromas as well depending on the type of wood used and the degree to which it has been charred or used to age other liquids.
Ranges from completely clear, to orange hued amber, to extremely rich caramel brown.
The flavor of whisky can be earthy, sweet, and grainy in the case of corn whisky, and it can be of all of the following and/or more: spicy, peppery, sweet, caramel, vanilla, toasty, woody, fruity, earthy, nutty, buttery, minty, herbal, dry, sweet, etc.
Whiskies are typically not lower that 40% alcohol by volume and many cask strength versions reach levels in the mid-to-high 60% range.