Sometimes I see questions or answers marked as "Community Wiki", or comments from other members saying that something should be Community Wiki. What does this mean, and how/when I should I be using it?
Why do we have Community Wiki?
One of the distinguishing characteristics of a wiki (such as Wikipedia) is that it encourages everyone to edit content. By marking your post as Community Wiki, you are inviting other members of the community to edit it - not just to correct spelling or improve formatting, but also for content and/or correctness.
What does it actually do?
Community Wiki mode lowers the reputation bar for other people to edit your post. If you take a look at your privileges, you'll see that the reputation required for editing Community Wiki posts is much lower than that required for editing other posts.
A side-effect of having a potentially large number of people collectively editing a single post is that no one person truly "owns" the content. As a consequence, Community Wiki votes do not affect the reputation of the author.
How does a post become Community Wiki?
There are several possible methods, originally outlined on Meta Stack Overflow:
Why do some questions show as Community Wiki?
When the Community Wiki feature was first introduced, authors could mark their questions as Community Wiki, which would force all answers into Community Wiki mode. This feature was eliminated in October 2010. Questions can no longer be marked Community Wiki, except by moderators. The feature is, of course, still available for answers.
Why can't we use Community Wiki for questions anymore?
The original expectation for Community Wiki questions was that they would result in "canonical" wiki-style answers, by taking away the primary motivation (i.e. reputation) for other members to post their own individual answers. Instead, they would collaborate on a single, thorough, excellent answer.
Unfortunately, that's not how things worked out. The feature led to mass confusion and frustration. Several people became fixated on the rep-immunity aspect instead of the collaboration aspect, and it became frequently used and encouraged as a justification for inappropriate questions that would normally be closed. Some members would be badgered into making their questions Community Wiki, even when they did not actually have the ability.
A contingent of the Stack Exchange community were upset with these perceived misunderstandings and abuses, and the Stack Exchange development team addressed their concerns by removing the feature from questions.
But what if we want to create a list or poll?
Don't. This site is intended for questions that can not only be answered objectively, but also voted on objectively. If a question encourages members to post or vote for their "favourites" then it does not belong here.
If you are absolutely certain that your question is of general interest and can be answered completely and objectively in a single post, then ask your question normally and start a Community Wiki answer to indicate that you want a single comprehensive answer and wish for other people to collaborate on it.
What if I see somebody else creating a poll? Didn't you tell us to flag these?
Yes, and you should continue to flag these list/poll questions for moderator attention if you believe that they are attracting or are likely to attract a lot of "cheap" answers. Moderators may still enforce Community Wiki mode as a kind of emergency measure to prevent "rep farming".
However, it is more important to close questions that don't belong here, assuming that you have this privilege. The biggest problem with polls is not the amount of reputation gained from them, it is the quantity and quality of answers they attract and the amount of noise they add to the site. If you feel that a question is too broad or open-ended to justify any reputation being gained from answers, then please vote to close as Not A Real Question instead of (or in addition to) flagging it.
What if somebody leaves a comment that my question should be Community Wiki?
Flag their comment. A ♦ Moderator will delete it.
Is there anything else I should know?
A few miscellaneous items: