API:Meta

Meta queries return information which is not associated with pages.

siteinfo / si
Returns overall site information.

Parameters

 * : Which properties to get
 * : General site information (default)
 * : A list of all namespaces
 * : A list of all namespace aliases
 * : Site statistics à la Special:Statistics
 * : A list of all interwiki prefixes and where they go
 * : Get information about the database server with the highest replication lag
 * : Criteria to filter the interwiki map by (ignored if  isn't set)
 * : Only list local interwiki prefixes
 * : Only list non-local interwiki prefixes
 * : Show information about all DB servers (ignored if  isn't set)

userinfo / ui
Returns information about the currently logged-in user.

Parameters

 * : Which properties to get
 * : Whether the current user is blocked, by whom, and why
 * : Whether the current user has new messages on their user talk page
 * : Which groups the current user belongs to
 * : Which rights the current user has
 * : Which preferences the current user has
 * : The number of edits the current user has made
 * : Rate limits applying to the current user

allmessages / am
Lists the contents of all (or a few) interface messages.

Parameters

 * ammessages: Only list these messages
 * amfilter: Only list messages whose name contains this string
 * amlang: Translate messages to this language

Example
As you can see, querying a nonexistent message does not do what one might expect: it returns  as the message content.