Subpages are groups of pages that share a common prefix, which itself is another page. While this is also possible with "classic" wiki, by using names like SomeTopicSubTopic, the use of SomeTopic/SubTopic allows better navigational support, and you can omit the common prefix when linking from the parent page to the child page.

Thus, by using "/" to concatenate several WikiNames, you can create arbitrarily deep hierarchies (within limits, especially the length of filenames on your system). In reality, subpages are normal pages that contain a "/" in their name, and thus they are stored besides all other pages in the file system. Subpages are a configurable feature, but they're on by default and it's recommended that you keep it that way, since the help pages themselves use this feature.

 * HelpOnEditing/SubPages
 * [wiki:HelpOnEditing/SubPages This very page]
 * [wiki:Self:HelpOnEditing/SubPages This very page]
 * /ThirdLevel
 * [wiki:/ThirdLevel A page below this one]

• HelpOnEditing/SubPages
• This very page
• This very page
• /ThirdLevel
• A page below this one
  
  Please do all of us a favour and don't create the /ThirdLevel pages!