There is one main page, where the main text of article is to be found, and a number of subpages which are associated with the article, and logically are parts of it. These subpages contain ancillary material which we don't wish to clutter up the main article page with, such as lengthy bibliographies, advanced material which will only be of use to specialists, etc.
A subpage is basically just a ordinary MediaWiki page with a "/" in its title. Subpages, which have names of the form "Foo/Bar", are all associated with a particular top-level page, which is the one with the name consisting of the part of the subpage's name before the "/" (in this example, "Foo"). (And of course the process is recursive, so that "Foo/Bar/Zap" is a subpage of "Foo/Bar".)
Other than being related to a particular top-level page, there is almost no real difference in how they are actually handled by MediaWiki. (See this page for more details. In fact, depending on how MediaWiki is configured, and which namespace they are in, the MediaWiki software may not even recognize subpages at all in that namespace.)
On Wikipedia, sub-pages are turned off in their main 'article' namespace. As we developed the subpage structure of Citizendium, it became clear that for technical reasons which need not concern us here, it would not work without subpages being enabled in the main namespace.
One advantage Wikipedia gains by having subpages turned off in the main article namespace is that it makes it easier to use a backslash in an article name, where that would be natural (such as Apollo Command/Service Module). In Citizendium, to make our current subpage mechanisms work, we have to make such page names into redirects; for example, GNU/Linux.
- CZ:Namespaces - A list of all CZ's namespaces