Here’s a good tutorial from ExtremeElectronics.co.in explaining the types of stepper motors, their uses, and how to interface them with an AVR microcontroller.
It includes the xstepper code library for use with the AVR Atmega16, and demonstrates how to interface the MCU and the motor using a ULN2003A high-voltage high-current Darlington transistor array.
This tutorial is comprehensive and provides beginners with the theory and sample code to get started with a simple stepper motor project.
Wow, this tutorial is really bad. First, I was willing to ignore the bad English, since there are many languages in the world and good technology comes from many places where English is not the first language. But then I see examples claiming that a permanent “North” magnetic pole would be attracted to an electromagnetic “North” pole. In actuality, opposite poles (north and south) attract. A stepper motor would really be broken if you tried to pull around a permanent N via electromagnetic N poles.