A Wiki Charter is a document that sets guidelines for community collaboration and respectful, productive activity on the wiki. The charter should be created at the start of a new wiki so that new members have the benefit of community guidelines as soon as they begin to collaborate.
The charter is a good way to introduce people to the wiki so that they can become exemplars of good wiki practice right from the start.
In communities where the membership is mostly or all virtual, the charter is especially important because people only know each other online. Groups where there is some element of physical interaction typically have fewer problems with online collaboration because the basic structure of community is already in place.