Tessellations.org is a good site for exploring the art form...I especially like the "Do-It-Yourself" links where the step-by-step directions and examples model the creative variety of transformations that tessellated paterns may produce. Check it out!

http://www.tessellations.org/index.htm

Tessellations: How to Create Them - Jim McNeill
This video produced by Crystal Productions, features tessellation artist Jim McNeill. The video includes basic concepts, historic examples and step-by-step demonstration of how to create three types of tessellations. It is well paced, well filmed and very easy to follow. It is appropriate for use with a wide range of ages. I recommend it over others I have seen on this topic.

Tessellation World of Makoto Nakamura
http://www.k4.dion.ne.jp/~mnaka/home.index.html
http://www.tessellation.info/?open=general/artistinfo.php&lang=english&style=layout&people_ID=5