Drupal doesn’t have a built-in wiki, but there are different options for incorporating a wiki with a drupal site.
One option is to use a separate wiki engine such as the popular mediawiki. There is an AuthDrupal extension for mediawiki that makes mediawiki use the drupal database for user account authentication.
Or you can create a wiki entirely within drupal itself. There are different options here as well, but some modules that you can use include:
- wikitools – you create a new content type (called ‘wiki page’ for example), and set wikitools to use it as the wiki type. Wikitools lets you set it so anyone you wish can edit the page, and it prevents duplicate pages from being created. Make sure you check ‘create new revision’ as the default option for that content type. You can also use the built-in book content type for wiki pages, if you want to enable hierarchically organized wiki pages for example as supported by the Atlassian Confluence wiki engine. See also these notes on making a drupal wiki with the book module.
- talk is a nice module that puts the comments on a separate talk page, just like with most wikis.
- freelinking lets people create [[bracket style]] links to other pages. If the page name doesn’t exist, freelinking displays links to search for or create the page.
- interwiki lets you create [single bracketed] links to other wikis or sites, such as for example: [map:Logan, UT] or [dictionary:someterm].
- video_filter lets people easily create embedded video links with syntax such as [video:http://www.youtube.com/watch?v=SOMEID%5D
- Other modules that add wiki-like touches include tableofcontents, recent_changes, and trash. Although in the case of recent_changes, I found the built-in “tracker” view was just as good at showing recent activity on the drupal site.
- If you want an input format that supports full mediawiki syntax, try for example pearwiki_filter, flexifilter (drupal 6 only), or the mediawiki module. I personally am fine with the WYSIWYG FCKEditor and IMCE modules instead, since special wiki syntaxes are difficult to master and can have the effect of alienating many users.
If you use pathauto and organic groups (og) you can have group-only wiki pages show up in a separate namespace. Also, recent versions of organic groups added a checkbox in the content types administration to set a content type as ‘wiki-like’, being editable by anyone in a group, which may mean wikitools is not even necessary if you only want wikis for groups.