From PostgreSQL wiki
Pages without a ':' in their name are in the main namespace. This namespace is for content pages; documents that aren't directly related to this may go in other namespaces. When pages are in another namespace, they are specified as namespace:page; i.e. this page is Help:Namespaces.
- Documentation about working with the wiki software. This could be mirrored from outside sites, or locally written.
- For descriptions of uploaded files or media files. You shouldn't create these directly; they are created when you click the Upload file link in the toolbox.
- Use this namespace to link to uploaded files directly, rather than through the description pages.
- Use this namespace to change the default system messages, See Help:System message on meta.
- Information about this wiki; i.e. policies that apply here. This namespace also has an alias, which is the name of the wiki installation.
- Each page has a corresponding discussion page. This can be used for feedback/comments about that page, or other local notes that another group may want to associate with the page, without modifying the document directly, or for any other additional information to associate with the document.
- This is used for meta-information that is to be transcluded into multiple documents, such as tags to mark the status of a document.
- For personal notes. Each User has a corresponding user page for their own information. Users can also create subpages, by using a / after their name.
- The discussion page on a user's page can be used for leaving messages. If this page is edited, the next time that user logs in they will see a box notifying them that they have new messages