A tessellation is a combination of one or more shapes that, when repeated, cover a surface with no gaps and no overlaps.
The word "tessellation" comes from the Latin word "tessella", meaning a small square tile used in ancient Roman mosaics.
A tessellation is also called a "tiling".
A commonplace example of a tessellation is a checkerboard.
It is also possible to tessellate non-plane surfaces, such as a sphere or a torus (doughnut).
If you want to see some tessellations, follow the links to Totally Tessellated and to M. C. Escher below.
You can go to Totally Tessellated, a terrific web site with lots of information and pictures.
You can see about M. C. Escher, an artist known for his tessellations.
You can see about software to help draw tessellations.
You can buy tessellation kits.