Wikis are a natural tool for collaborative genealogy efforts and for thousands of other online tasks. Several genealogy wikis already exist and I am sure that we will see more in coming months. I have written before about wikis. However, a new 4-minute video does a great job of explaining the basics of a wiki.
If anything, the "Wikis in Plain English" video is a bit too simplistic. However, it will get you started. You can view the video at http://www.commoncraft.com/video-wikis-plain-english.
Once you learn the basics in the video, you might want to read a more detailed explanation on Wikipedia at http://en.wikipedia.org/wiki/Wiki.
NOTE: Wikipedia is the largest wiki in the world.
You can find genealogy wikis at: