A Wiki is a collaborative web site. At the bottom of most pages, you will see a link "Edit this page" which you can use to do just that. Wikis are written is a simplified markup language. A description of the text formatting rules for this Wiki can be found at SyntaxRules. Or of course take a look at some other pages....
If you see links with a : in the middle, then that means it is an external link of some sort, mostly to other Wikis. The bit before the : indicates which site the page is hosted on. For instance, Wiki indicates the original Wiki and OddMuse is the Wiki for this specific implementation.
One of the most useful pages is RecentChanges. Its probably a good idea to look here quite often - perhaps even bookmark it and use it as your starting point.
Wiki:BackLinks are very useful - click on the title of a page and it shows all the pages that reference it. Try it out!
For more introductory stuff please see these pages: