A tessellation is a repeating pattern of distinct shapes. Think of his interlocking "tiles" of lizards or polygons. Creating such things requires a sound knowledge of geometry and not a little artistic flair. You can stare at these things for hours! Below are a couple links in case you want to do just that.
http://www.geocities.com/SoHo/Museum/3828/tessellations.html
http://library.thinkquest.org/16661/escher.html