Who's your Daddy?
Vanilla uses an extremely simple parent > child architecture for all of its pages. Parent pages are ones like those found on the site's main navigation menu (Home, Features etc.). These are also called top level pages. Pages that sit below these are classed as 'child pages'. A parent can have any number of children. Child pages can also have children, which in turn can have their own, and so on.
If a child page's immediate parent page is deleted, it does not disappear, but instead becomes an 'orphan' page. When in admin mode, orphan pages are marked as such on the site map.