A. Deep Background
Right off the bat, let me say that you don't need to read this appendix. It's deep background, the kind of detail that you might wish to delve into when you're attempting to understand how iPhoto works, perhaps because you've just printed a photo and you're unhappy with the results.
The following pages contain "Understanding Aspect Ratios," "Understanding Resolution," and "Understanding Color Management." Each of these discussions examines an aspect of digital photography from which iPhoto, for the most part, tries to shield you. That's great most of the time, but if you're trying to understand how cropping removes information from a photo, thus making it print at a lower quality, you'll want to come here for the explanation.
Lastly, although I've called this appendix "Deep Background," these topics are so complex that entire books have been written about each one. If these discussions leave you with more questions, I'd encourage you to visit a library or bookstore and browse its collection of books on photography, digital imaging, and pre-press. I especially recommend Real World Scanning and Halftones, Third Edition, by David Blatner, Conrad Chavez, Glenn Fleishman, and Steve Roth.