What is a community wiki and when is a question marked as a community wiki?
When questions are marked “community wiki” it means that other members of the community can freely edit them. The new Discuss is a lot like a wiki since everything that is entered and/or editted is fully versioned. If someone makes changes that are incorrect or inappropriate we can always roll back to an earlier version. Community wiki posts exist to allow people to share their knowledge collaboratively. When a question has no definite answer like “How to get better at …” and other similar questions, they should be marked as community wiki.
It’s also worth noting that questions (and answers) marked as community wiki do not collect reputation points when they’re voted up or accepted.
The detailed explanation to this can be found here: http://discuss.codechef.com/questions/10899/what-is-a-community-wiki