A Wiki is a documentation to which each and every user can contribute freely.
The aim of a Wiki is to not use much special technical syntax elements which would create difficulties.
Instead, it clearly aims towards "keep it simple".
Of course, it has special elements for making text bold, like this text, or cursive, like this one, but normally, you can simply just hit the "edit" button in a wiki's menu and start writing documentation or improve existing one.
For more information about Wikis in general, see Wikipedia: Wiki.