Wikis allow users to maintain unstructured content very quickly and easily. Wiki applications add structure to wiki content.
Wikis engines that have a plugin API can be extended with application functionality, such as calendaring, spreadsheet calculations, charting and more. Those applications are created by programmers and are installed by the wiki administrator.
Structured wikis such as TWiki, XWiki and TikiWiki CMS/Groupware go a step further and enable users to create their own wiki applications. Those applications are created by using the wiki's markup and scripting language in wiki pages. Structured wikis provide database-like manipulation of fields stored on pages, and offer a query language to embed reports in wiki pages.
A structured wiki lends itself to support evolving processes at the workplace. First by enabling employees to document processes in the free-form wiki way, with linked pages maintained collaboratively. Secondly, by creating structured wiki application with forms, queries and reports that automate those processes.
Wiki applications are also called situational applications because they are created ad-hoc by the users for very specific needs. Examples include call center status boards, to-do lists, sign-off sheets for export compliance, inventory systems and more. Applications are typically fairly simple, although there are also more sophisticated applications, such as employee handbooks, bug trackers and blog applications, to name a few.