This site from the Georgia State University provides a good overview of ferromagnetic materials, introducing concepts such as magnetic domains, hysteresis, the curie temperature, and magnetostriction.

A great introduction to magnetic domains, including pictures which illustrate the effect on domains caused by external applied magnetic fields.

This site from Wikipedia provides a wonderful in-depth explanation of ferromagnetism, covering the atomic behavior which is responsible for ferromagnetic properties. Also introduces the concepts of magnetic domains and the Curie...

This site is an introduction to magnetic domains, including a picture showing distinct domains taken through a process called "Magnetic force microscopy."