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. 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.
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.
AttachFile: attach files to a page, see /AttachFile for more details.
LocalSiteMap: show nearby links of the current page, i.e. list all referred-to pages, and in turn their references, to a certain limited depth.
SpellCheck: call the spell checker for the current page; see HelpOnSpellCheck for more.
The following is a list of internal actions that are used to implement the various icons and links at the top and bottom of pages, or supplement certain macros.
fullsearch: this action is triggered by clicking on the "Go" button of the FullSearch macro (?action=fullsearch&value=HelpOnActions).
titlesearch: this action is triggered by clicking on the "Go" button of the TitleSearch macro (?action=titlesearch&value=HelpOnActions).
inlinesearch: this implements the inline search for the form fields at the bottom of each page (note that this uses POST requests, so you won't normally see this action in URLs). [0.11]
subscribe: subscribe to a page. [0.11]
bookmark: set bookmark for RecentChanges.
formtest: used for development. [0.11]
rss_rc: generate a RSS feed of RecentChanges. [0.11]
chart: display charts. [0.11]
dumpform: used for development. [0.11]
export: export the wiki content. [experimental]
content: for transclusion into static web pages, this action emits the pure page content, without any <html>, <head>, or <body> tags. [1.0]
links: generates a list of all pages and the links on them, just like LinkDatabase. [1.0]
xmlrpc: Wiki XML-RPC interface, see XmlRpcToWiki. [1.0]