The information I think you're looking for is available via the API's sitematrix action:

The Wikimedia projects are typically referred to as "projects" (in the context of their content and contributors' day-to-day work) or "sites" (usually in the context of their technical infrastructure and operations).  A namespace is a concept used in MediaWiki to group pages by type at a high level.  Examples of namespaces include "Talk" for discussion pages about articles, or "File" for pages about uploaded media files.  See this page for a fuller discussion:

-mdh