Actions are tools that work on a page or the whole wiki, but unlike macros do not add to the page content when viewing a page, but work on that page content. They either produce some output based on page contents (navigational actions like searching) or implement functions that are not related to viewing a page (like deleting or renaming a page).
There is a set of system-defined actions, which includes page editing; you can extend the set of known actions by your own ones. See HelpForDevelopers for more on that.
The following actions are added to the list of user-defined extension actions at the bottom of each page (or in the "More actions" menu). This happens to any mixed-case extension, for other actions (all lower-case) see the list further down this page. Some of these action might not be available for your wiki site.
AttachFile: attach files to a page, see /AttachFile for more details.
DeletePage: Delete a page, after you confirmed it; deletion means a final backup copy is created and only then the page is deleted, i.e. you can revive the page later on (as long as the backups are not physically deleted).
LikePages: list pages whose title starts or ends with the same WikiWord as the current page title.
SpellCheck: call the spell checker for the current page; see HelpOnSpellCheck for more.
titleindex: Implements the listing of all page names as text (append ?action=titleindex to your wiki address) or XML (?action=titleindex&mimetype=text/xml); the main use of this action is to enable MetaWiki.
bookmark: set bookmark for RecentChanges.
raw: send the raw wiki markup as text/plain (e.g. for backup purposes via wget); SystemInfo?action=raw will show the markup of SystemInfo.
You can use it to generate DocBook markup from your pages. Just append ?action=format&mimetype=xml/docbook to the address. Note that you need to install PyXML to use it.
rss_rc: generate a RSS feed of RecentChanges.
links: generates a list of all pages and the links on them, just like LinkDatabase.
xmlrpc: Wiki XML-RPC interface, see XmlRpcToWiki.