All pages on Wikipedia are of two types: About two million articles constitute the encyclopedic content, but ten million project-related pages also exist. What are these pages? Will you see them if you just look something up? If you find them when using a search engine, should you ignore those hits?
Wikipedia's readers should recognize that some Wikipedia pages are not articles, but they do not need to have any particular understanding of the non-article pages and can ignore them freely. On the other hand, involved editors should understand the different types of pages—their purpose and the way they help grease Wikipedia's wheels. The project-related and administrative pages are not as glamorous as articles, but they're of no less importance when it comes to understanding what happens in practice on the site.
These extra pages come in several varieties. Non-article pages are devoted to the administration of Wikipedia, discussion of article content, technical infrastructure, descriptions of images, and the Wikipedia community.
Although they are not as widely known as articles, two of these page types—discussion pages and user pages—are actually the easiest places to start participating on Wikipedia.
The other kinds of pages are typically used as references and project coordination pages.
Each type of page is distinguished from every other type (including from articles) by a prefix; for example, discussion pages are prefixed with Talk:. This prevents "collisions" between similarly named pages, for example, [[Sorting]], which is an encyclopedia article about the process of arranging items, and [[Help:Sorting]], which is not an encyclopedia article but instead offers technical assistance about the sortable tables found on some Wikipedia pages.
Each prefix is actually an indicator that the page is inside a particular namespace. (A namespace is a kind of container for different types of content.) For example, in this full Wikipedia URL
http://en.wikipedia.org/wiki/Talk:Benjamin_Franklin
Talk indicates the namespace where the page exists, whereas Benjamin Franklin, separated from the namespace with a colon (:), is the page's name. If you were internally linking to this URL, you'd use the combination of the namespace and page name to properly indicate what page you meant: [[Talk:Benjamin Franklin]].
Articles, which exist in the so-called main or article space namespace, do not have prefixes:
http://en.wikipedia.org/wiki/Benjamin_Franklin
Benjamin Franklin is the full page name; the absence of a prefix tells you the page is an encyclopedia article.
All other types of content in Wikipedia exist in one of the other namespaces, which are indicated with one of 19 possible prefixes. Seeing a prefix before a title tells you that the page is likely part of the community or administration of the site (and therefore is not subject to the same content guidelines as articles).
The namespace also provides context and indicates the type of content that a page contains. For example, help pages contain technical documentation, rather than (say) encyclopedia articles or policies.
Although two pages in the same namespace cannot share a title, pages can exist under the same "name" in different namespaces. For example, the article [[Phoebe]] is about a personal name and is part of the encyclopedic content of the site. It is not the same thing at all as the page [[User:Phoebe]], which exists in the User namespace and describes an editor who uses this name as a pseudonym.
The lines between encyclopedia content, on the one hand, and the Wikipedia community pages, on the other, are extremely clear and are delineated with the use of namespaces. As implemented on Wikipedia, community namespaces do not always exactly correlate with a single specific type of content. For instance, whereas only user pages are in the User namespace, you may find various pages such as technical documentation, community projects, and policies in the Wikipedia namespace. All of these pages, however, will have something to do with the running of Wikipedia.
Wikipedia has 20 built-in namespaces. These occur in pairs (for example, User and User_talk); there are nine such pairs, including the main namespace, where page names have no prefix, and two special namespaces, Special and Media. A namespace prefix must be kept when linking to a page. The prefix always comes before the page name and is separated from it with a colon.
For reference, the following namespaces exist:
The main or article namespace has no special prefix. This namespace is where all regular articles (all the "encyclopedic" pieces of the encyclopedia) exist. Pages in this namespace can be linked to internally with simply their name: [[pagename]].
The Wikipedia namespace is what could be called the project page namespace. It is for pages that are specifically about running Wikipedia and meta-level subjects related to the project. For example, the Community Portal can be found at [[Wikipedia:Community_portal]] and is meant as a place for the Wikipedia community to gather; [[Wikipedia:Statistics]] and its talk page, [[Wikipedia_talk:Statistics]], are meant for describing and discussing the project's statistics. Policies, procedures, guidelines, community projects, and many help pages all exist within the Wikipedia namespace. The Wikipedia namespace may sometimes be abbreviated to WP, enabling shortcuts to be set up. For instance, [[WP:ARB]] redirects to [[Wikipedia:Arbitration_Committee]].
The User namespace refers to user pages or pages that have been set up by individual editors to describe themselves, for example, [[User:Jimbo Wales]]. By custom, your user page is available when you register a username.
The Help namespace refers to basic documentation and help pages for using and editing Wikipedia. The prefix for these is simply Help:. Most of the project documentation pages are here or in the Wikipedia namespace.
The Category namespace is a major part of expertly using Wikipedia; we discuss categories at length in Chapter 3, Finding Wikipedia's Content and Chapter 8, Make and Mend Wikipedia's Web.
The Image namespace is prefaced by Image: and is used for describing and attributing images (for example, [[Image:White shark.jpg]]). If you upload any image or other media file to Wikipedia, one of these pages will be created. The Media namespace is prefaced by Media: and is used for a link directly to a media file, rather than its description page. Details are in Chapter 9, Images, Templates, and Special Characters.
The Template namespace is prefaced by Template: and is used exclusively for templates that are transcluded or substituted into an article. You'll find more on templates in Chapter 9, Images, Templates, and Special Characters.
The Portal namespace is for portal pages that collect articles on a particular topic; this is special to Wikipedia and not generally for MediaWiki. For more on portals, see Chapter 3, Finding Wikipedia's Content and Chapter 7, Cleanup, Projects, and Processes.
The Talk namespaces contain all the discussion pages. Except for special pages, every namespace has an associated Talk namespace, designated by adding talk: after the normal namespace prefix. In this book, we write these compound names with an underscore to be clear, but you can always use a space. The Talk namespace associated with the main article namespace simply uses the prefix Talk:, for example, [[Talk:Mathematics]]. The Talk namespace associated with the User namespace, however, has the prefix User_talk:. Similarly, Wikipedia namespace discussion pages are in the Wikipedia_talk namespace, so the discussion page for [[Wikipedia:No original research]] is at [[Wikipedia_talk:No original research]]. Generally, pages in the Talk namespaces are used to discuss changes to their corresponding page; however, pages in the User_talk namespace are used to leave messages for a particular user. The User_talk namespace is special in that, whenever a user's talk page is edited, that user (if logged in) will immediately see a message informing them that they have new messages.
The Special namespace refers to pages that are autocreated by the site's software on demand. These pages are not editable in the usual way and are generally either tools or automatically generated variable lists, such as a list of all pages on the site. See [[Help:Special page]] for a list.
The MediaWiki namespace is used for certain site messages along with a few other areas to define shortcuts and other text strings used around Wikipedia (for example, [[MediaWiki:Disclaimers]]). These pages are not usually editable by users.