Wiki is the short form of a Hawaiian word (wiki-wiki) which means quick. Wikis are websites that are collaborative in nature. Say you want to work on a web project with teachers in another state. A wiki would allow you to do just that. It makes a great collaborative tool for students too.
Watch this Video that explains how wikis work.
The most well-known example of a wiki is Wikipedia. Here is a Wikipedia page that gives suggestions about why you might want to use wikis in your classroom.
You might want to also visit Wikis in the Classroom as it gives some great information about how to set up a wiki.
Here are some examples of actual classroom wikis:
Terry the Tennis Ball is done by 3rd and 4th grade students.