Purpose of namespaces in different Wikimedia projects
Main namespace
Project namespace
Help namespace
Wikipedia
encyclopedia articles
information about Wikipedia—its policies, guidelines, rules, etc.—along with some help about MediaWiki that is particular to Wikipedia
copied from Meta-Wiki's help namespace
Meta-Wiki
meta-pages: information relevant to multiple Wikimedia projects
pages related to Meta-Wiki itself, not to other Wikimedia projects
documentation of the basic, technical features of MediaWiki
MediaWiki.org
all general information about the software
organisation of MediaWiki.org (this namespace should be as small as possible)
set of basic help pages for users of existing wikis (formatting, etc.), aiming for a complete English version that can be shipped with the MW distribution (unlike elsewhere, contributions in this namespace are licensed in the public domain)
Additional namespaces
MediaWiki.org has two additional namespaces:
Manual :All technical or more detailed stuff behind the surface, that is not part of the basic help pages (full handbook, mainly for the ones who want to set up an own wiki).
Extension :All extensions and directly related stuff.