In the Apple itunes store you can download free apps to your device to read books. So I have the Kindle app to view books I purchased online at Amazon on my iPad or iPhone, and even on my PC.
iBooks is Apple's own eBook reader. I actually like how I can organize books in the iBook app.
The Kindle app is probably available on other smart phones too, Amazon probably advertises which devices you can download it to.
I didn't even know you could see a Kindle in a store in the states.