Voronoi tessellation is a mathematical technique that is often useful in procedural generation. A Voronoi cell is the space that is closest to a particular point. It’s an effective and natural-looking way to divide up a plane.

The way the distance is calculated affects the shapes of the cells; most common is Euclidian distance, but different distance functions will produce different results. The points can also be weighted, so that some have more pull than others.

http://en.wikipedia.org/wiki/Voronoi_diagram

The Voronoi wiki has some useful information:
http://www.voronoi.com/wiki/index.php?title=VoroWiki_main_page