Everything you ever wanted to know about scanning, resolution, scaling and resampling by Wayne Fulton.
"The purpose is to offer some scanning tips and to explain the basics for photos and documents. It is about the fundamentals of digital images, about the basics to help you get the most from your scanner. How it works, for those that want to know".
"Included here are the general questions that we've all asked about digital images, but unfortunately, that other sources don't answer. The material is about the basics, and is appropriate for the beginner. The content is certainly not superficial, but it is not at all difficult either, it is just simply about how it works. It describes in plain language the things we need to know to be efficient and get the most from our images, in the various ways that we can use them". http://www.scantips.com/
This is must know information for anyone working with digital images..
PS: Move down the page until you see "Contents" and "Scanning 101 - The Basics". Also check out "Scanning 201" covering more advanced topics and try the Scanning/Printing Calculator.