I lived in the U.S. for a few years and, in that time, I opened an account with Bank of America. It was just by happenstance that I discovered on the internet one day that B of A and Scotiabank in Canada have some kind of partnership arrangement.
One of the benefits of B of A and Scotiabank being in cahoots is that you can use your B of A ATM card at any Scotiabank ATM in Canada without paying a service charge to either bank. Similarly, you can use a Scotiabank card at any B of A ATM without getting charged.
I still have my B of A account and I have used the ATM card at other bank ATMs in some fairly obscure places around the world. It works, but B of A charges $3 or so for the privilege, and usually a local service fee also attaches.
There are probably other banks that are in bed together under similar arrangements and, perhaps the whole world - excluding me - is aware of the details. In fact, it may be that what I am reciting here is so open and notorious that other C-Brats will be shaking their heads and wondering why I bothered with this post. To me it came as a revelation that I could use my B of A card in Canada w/o charge.
If you use the card to withdraw in Canada, as Dave said, the ATM will spit out the local currency. You can withdraw any amount you like, in increments of $20. If you tell the machine you want $100, you will receive $100 Canadian. When you go online and look at your account balance, you will see that your U.S. bank withdrew from your account the U.S. equivalent. So, if a Canadian dollar costs 90 cents U.S., you'll see that $90 got taken from your account. My experience has been that when you use a B of A card at Scotiabank, the exchange rate is fairly decent, sufficiently so that it's not worth shopping around for a better rate.