Why answers or questions are converted to "Community wiki"? as one of my answer and one question are changed to Community Wiki.
Any question or answer edited a sufficient amount of times is automatically turned into a Community Wiki - meaning that the question or answer is now owned by the Community so the originator will no longer receive reputation points (up or down) from that answer.
A question and all it's answers will also turn to Community Wiki if there are too many answers. This is partly to discourage the influx of reputation caused by more vague or list style type questions. (eg. Terminal Tips for Mac OS X)
Community Wiki posts are there to help encourage users to edit and update the answers on the Stack Exchange site to keep it relevant and accurate.
For more of the nitty gritty details (including the number of times items need to be edited) check out the Community Wiki FAQ on meta.stackoverflow.com: What are "Community Wiki" posts?