Hierarchy levels
Many conventional websites use very branched structures with many hierarchy levels.
In wikis linking is heavily used. A simpler structure is often used to simplify matters:
- all important topics are located in main pages.
- only if a topic has many subtopics (which aren't main topics), subpages are used.
Sub pages are named as MainPage/SubPage
Conventions
A wiki gives much freedom to the users; there aren't input masks like in database applications, but you can, in principle, do whatever you want and do it however you want.
Of course, you should avoid that to prevent complete chaos. 
That's why it's usual for new wikis to think about structures and conventions, such as the:
- creation of a basic structure of pages.
- definitions of name conventions for page names.
decision to use CamelCase or free links?.
- definitions of important categories.
It's usual for older wikis to simply carry on using existing structures.