What are Community Wiki posts?
Community wiki posts are more easily editable by all users, and do not confer any reputation.
When should I make my answers Community Wiki?
When you want to enhance the "wiki" aspect of your post, so that it can be a continually evolving source of good information through repeated editing.
When you feel your post would benefit from less concern about voting affecting the reputation of those participating in it.
How do Community Wiki posts work?
You can choose to make any answer you own a community wiki by ticking the checkbox under the edit area to the right.
Moderators can also choose to convert posts into community wiki mode if they feel it is appropriate for the question or answer, generally after a discussion with the community and affected individuals. Once a post is made community wiki, that mode can only be removed by a moderator under exceptional circumstances.
Also see What are “Community Wiki” posts?