Hi there

think of a wiki as an organic node based site. You write a page or article. lets say that you write one on wild life in the woods ans mention squirrels. If another user has access to edit they could put [[squirrels]] and creat a link to a new topic allowing them to put more detailed information about squirrels, note that while a link may be created to a topic the topic is not entered into the index until it has some content. If you mention that topic in another you can link to it in that same fasion of place square brakets around the name/word.

In regard to permissions there is a bug in the current wiki so if you want users to be able to edit it currently assign them to a rle that has edit privileges to the wiki module, don't worry the topic history allows you to restore from mistakes etc...
There should be a new version out soon but I use the wiki for a number of internal clients the example I use is http://ewds.strath.ac.uk/Development.aspx

hope that helps there are further instructions shown when you edit the is a help section beneath the rich text box which you can expand.

John