Wikimedia/What are namespaces?
Namespaces are a system of categorization of content using the MediaWiki software. It clusters together similar content. For instance, every wiki has a "user" namespace that consists of pages about users, talk pages to contact users, and other materials generated by users to help them in their work, such as drafts. The user namespace, like most other namespaces, is an identifying word followed by a colon--User:.
The one namespace that doesn't have an identifying word is the "main" namespace, where the word is absent and the colon is optional. In Wikipedia, this main namespace is reserved for articles. In Wikibooks, for books.
There are many namespaces, which differ according to which project you are using in the Wikimedia universe.