There is a definition of "Community Wiki" available on the [Stack Overflow meta site][1]. As far as I know the information there applies to all other sites built on the Stack Exchange engine.

As I understand it a community wiki question is owned by the whole community instead of an individual user. As a result a lower bar is set for the reputation score needed to do certain things (like edits), and up/down votes do not have the affect reputation scores they would on a normal question.

**Update:** As [noted here by Erigami][2] and [this answer][3] on the Finding community wikis question, a list of community wiki questions can be obtained by using the search term "wiki:1". Here's a link to the results of that search on the parent site:

  http://ubuntu.stackexchange.com/search?q=wiki:1

<strike>I am not aware of any method to get a definitive list of questions that have been set to community wiki.</strike>

  [1]: https://meta.stackexchange.com/questions/11740/what-are-community-wiki-posts
  [2]: https://meta.askubuntu.com/questions/154/where-are-the-community-wikis/167#167
  [3]: https://meta.askubuntu.com/questions/160/finding-community-wikis/166#166