You might ask, "What's a wiki?" It's an online encyclopedia with articles contributed by many different people. The best known is probably wikipedia.org, the hugely popular encyclopedia with more than ten times the articles of any printed encyclopedia.
Familysearch.org has begun a “wiki” devoted to genealogy—primarily “how to” articles. You may view the articles or contribute your own at http://wiki.familysearch.org. See the news item about this in Eastman’s newsletter at http://blog.eogn.com/eastmans_online_genealogy/2009/09/familysearch-wiki.html where he gives a more complete description.