National Council of Teachers of Mathematics
The Math Forum: What Is a Tessellation?
The Math Forum defines and illustrates geometric tessellation.
Other
Porcelai Nia: Tessela
Tessellated geometric patterns in art are discussed and illustrated in this resource.
Wolfram Research
Wolfram Math World: Semiregular Tessellations
This one page Mathworld site has a number of examples of semiregular tessellations. A definition and a small amount of related information is given at the bottom of the page.