A wiki is like a research website in the context of one group. The wiki can contain nearly countless articles.
How is a wiki different from other applications?
A blog's authors are appointed in the blog creation screen under "Authors." A forum is crowd-sourced (like a wiki); however there is no "I have this question too" function in the wiki. A gallery is composed of files, which may be uploaded or downloaded. Wikis allow inserting / drag & drop image upload to an article, but don't allow downloads.
A wiki is composed of articles written by community members. The idea of a wiki is that it becomes reference material for the community.
Wiki articles have a star rating system.
A wiki article has a revision tracking system that lets you revert to a previous version or compare two versions.
How do I create a wiki?
Please refer to the What is an application? article for instructions.