Which information to get:

general

Overall system information.

namespaces

List of registered namespaces and their canonical names.

namespacealiases

List of registered namespace aliases.

specialpagealiases

List of special page aliases.

magicwords

List of magic words and their aliases.

statistics

Returns site statistics.

interwikimap

Returns interwiki map (optionally filtered, optionally localised by using $1inlanguagecode).

dbrepllag

Returns database server with the highest replication lag.

usergroups

Returns user groups and the associated permissions.

libraries

Returns libraries installed on the wiki.

extensions

Returns extensions installed on the wiki.

fileextensions

Returns list of file extensions allowed to be uploaded.

rightsinfo

Returns wiki rights (license) information if available.

restrictions

Returns information on available restriction (protection) types.

languages

Returns a list of languages MediaWiki supports (optionally localised by using $1inlanguagecode).

skins

Returns a list of all enabled skins (optionally localised by using $1inlanguagecode, otherwise in the content language).

extensiontags

Returns a list of parser extension tags.

functionhooks

Returns a list of parser function hooks.

showhooks

Returns a list of all subscribed hooks (contents of $wgHooks).

variables

Returns a list of variable IDs.

protocols

Returns a list of protocols that are allowed in external links.

defaultoptions

Returns the default values for user preferences.