General Actions:
As you've learned from the Programming feature it's possible to put code inside XWiki pages. You can combine several pages together to form an application. In other words an application is simply a self-contained set of XWiki pages. You can export these pages into a single archive file named a XAR (it stands for XWiki Archive) which can then be imported again into other wikis. This is a very nice way to create reusable applications.
Applications offer a powerful way to create advanced content within wiki pages.
Check the Extensions wiki which lists all applications available that you can install in your wiki.
Screenshot of the Calendar Application

Screenshot of the Meeting Manager Application
Screenshot of the Bulletin Board Application

Screenshot of the Polls Application

If you'd like to create new applications with XWiki our Developer's Guide will help you through the process.
If you are interested in participating to the XWiki project development itself or in building it from sources, you should refer to the XWiki development community.
If you still have questions you may subscribe to the mailing lists.