There is a definition of "Community Wiki" available on the Stack Overflow meta site. 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 and this answer 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
I am not aware of any method to get a definitive list of questions that have been set to community wiki.
