A space is a collection of pages
Pages are organized by subject into spaces. For examples, spaces could be created to represent projects, customers, or organizational units (departments).
Within each space, page are organized in parent-child relationships. A page may have only one parent (or no parent), and zero or many children. Children of the same page (called siblings) are sorted alphabetically by default, but siblings can also be manually ordered. Top-level pages have no parent. A top-level page and its children (and their children, and so on) form a tree of pages. A space may have more than one top-level page, so a space can have multiple trees. It's easy to see that the trees don't overlap (they don't share pages). Finally, each space has one home page (a default landing page), usually a top-level page.
News pages are also organized under spaces. But unlike regular pages, news pages are organized by date and time (no parent-child relationships).
Spaces are the best level for defining user-access permissions, such as who can read versus edit the pages in the space. So a subject might need to be split into more than one space to support different security levels.
Example
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Aliquam fermentum vestibulum est. Sed quis tortor.