borrowed from a friend on another list:

Whiskey is any liquor distilled from the fermented mash of grain.

Bourbon: not less than 51% corn mash, plus malt and rye

Irish: technically, any whiskey made in Ireland, but generally meaning a barley mash

Scotch: single malt whiskey (but of course there are blends), generally distilled from barley mash
I rather like a nice Bourbon, but I prefer a good single malt. Currently one of my favorites is The Balvenie 12 year Double Wood... the first 6 years are spent in a nice oak cask that previously held bourbon (imagine that) and the last 6 years are spent aging in a very good Spanish oak that previously held Sherry... adds some interesting contrasts. Very smooth with a decent woody taste and a hint of licorice while keeping a very full flavor. Ah, I think I need to open a new bottle tonight

oh and the Canadians also spell it whisky