See MetaWikipedia:Help:Namespace for the full story. Here is a summary with some details for this site.
|The "nameless" namespace is used for most articles, such as Main Page.
|Each logged-in user may have a user page. The user is generally notified when a message is added to their Talk page. User pages (and most Talk namespaces) may have subpages as needed.
|Uploaded images and sounds, and any wikitext about them. See Help:Image for information about putting images (and media: links) into wikicode.
|Categories of pages. A category link anywhere on a page puts the page into the category. To stop this and link to the category itself, use a leading colon: [[:Category:Chessboard]]. Moving a category means editing every link to that category, so choose category names with care.
|Templates are substituted into other pages. The talk pages also include documentation for the template, if any. See Help:Template.
|Also named "IcehouseOrg" on this site. Pages about this site itself, and how things work here, including policy pages.
|Pages about how to use the wiki, many imported from Meta. Some of these are customized for this project.
|Generated by the server as needed, such as Special:Recentchanges. No talk pages, not movable, not searchable.
|Fake namespace, used to link to an image or sound directly (as opposed to linking to the item's page). See Help:Image. No talk pages, not movable or searchable (but the corresponding image: item may be moved or sought).
|The messages used in the wiki itself, such as in the toolbox on every page. Mostly, these are not editable (or usable) by ordinary users, but everyone is welcome to make requests on the Talk page. Not movable.