From the Wikipedia:
A pattern language is a structured method of describing good design practices within a particular domain. ... Pattern languages are used to formalize decision-making values whose effectiveness becomes obvious with experience but that are difficult to document and pass on to novices. They are also effective tools in structuring knowledge and understanding of fundamentally complex systems without forcing oversimplification — including organizing people or groups involved in complex undertakings, revealing how their functions inter-relate as part of the larger whole.
WikiWikiWebs are ideal tools for writing pattern languages. The world's first wiki was the Portland Pattern Repository at http://c2.com/cgi-bin/wiki?WikiWikiWeb