When you ask a question, you have the option of making it a community wiki. Moderators also have the ability to force some questions to become a community wiki.

This is most useful for questions that are more like polls or lists. For instance, suppose your question is What measures can be done by individuals to reduce their impact on climate change? This question is on topic but there is no "right" answer. It makes more sense for everyone to contribute an item they found useful, and have the rest of the community vote those items up or down. Then the original question, or a designated answer (the accepted answer) can be edited to hold an ongoing list of the most useful items.

You only need 100 reputation to edit a community wiki, so a large proportion of the community can contribute.

Normally questions like these are very popular, and people vote a lot on all the answers they like, or even answers they find humorous. This would give people an incentive to only ask or answer "poll-like" questions because you could earn enormous reputation very quickly. To prevent this, community wiki posts have no effect on reputation.