The way I understand it …

• all whiskey must be distilled from grain (wheat, rye, barley and corn — not to mention the rare uses of oats and buckwheat)
• Bourbon is a minimum of 51 percent corn by law
• Scotch and Irish styles use barley as their base
• Canadian whisky is often mostly corn, but you can find some brands with a stronger rye base
• Scotch and Irish whiskies are made in beautiful copper stills often called pot stills
• American whiskey is usually made in more modern stills — known as column stills — containing copper plates inside
• distilling in a pot still is more labour intensive and much less efficient than a column still
• Bourbon and Scotch are typically double-distilled and Irish whiskey triple-distilled (Canadian law does not specify)
• the more times you distill, the less flavour you have
• Bourbon must be aged in new charred oak barrels by law (the used barrels get sent all over the world for aging other spirits, like other whiskies)
• some whiskeys are aged in used wine barrels
• “straight” Bourbon must be aged at least two years
• Scotch, Irish and Canadian whiskies are aged a minimum of three years
• American and Irish producers usually spell it with an “e.” Scotch and Canadian styles are typically spelled “whisky”
• the word whisky is derived from the Gaelic “uisge beatha” meaning “water of life”