The Index, accessed by the button in the lower right corner, gets a special page that shows the pages and namespaces within your wiki, organized as a tree and ordered by namespaces.
It may also be called by adding ?do=index at the end of your URL.
To view a specific branch of your wiki, you can also call ?idx=namespace_to_show.
In addition there are some very useful index plugins, with usage of JavaScript and other features
Be aware that it reveals namespaces (but not pages) that may be protected by the ACL. There is an option you can set in your configuration file to enable protected namespace hiding in the Index. This option is not turned on by default since this will hide accessible pages that are children of an otherwise protected namespace.