Christopher Alexander coined the term Pattern Language to emphasize his belief that people had an innate ability for design that paralleled their ability to speak.
Chris's book A Timeless Way Of Building is the most instructive in describing his notion of a pattern language and its application to designing and building buildings and towns.
He defines a 'pattern' as a three part construct. First comes the 'context'; under what conditions does this pattern hold. Next are a 'system of forces'. In many ways it is natural to think of this as the 'problem' or 'goal'. The third part is the 'solution'; a configuration that balances the system of forces or solves the problems presented.
Pattern Language Rules:
You will probably need one window open for writing your page, and at least one more to jump back and forth between references.
Ward Cunningham created Wikiwiki as a way to express pattern languages effectively; the Portland Pattern Repository has hundreds of patterns for a variety of things, including many for extreme programming.