a wiki is a collection of spaces
Your wiki makes all your spaces (and their contents) available to any web browser, as long as the user's permissions allow it. You can think of the wiki as a single web site, or a collection of web sites. It's possible to change the look-and-feel for each space, different sections within a space, or even different users. Just about anything is possible with [custom themes].
Any setting that affects the entire wiki is usually referred to as global. For example, certain user-access permissions apply across the whole wiki, and these are called global permissions. More on permissions in a minute . . .
Example
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Aliquam fermentum vestibulum est. Sed quis tortor.