The Index, accessed by the button in the lower right corner, gets a short overview on all existing pages. It's organized as a tree, ordered by namespaces.
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.
In addition there are some very useful indexmenu plugins, with usage of javascript and other features