A wiki is a collaborative information repository. For the most part, anyone that can access the site can add and modify information contained therein. The safeguard to this openness is that each revision is tracked and can be rolled back if needed.
It's a really powerful tool if a lot of people pitch in and add their collective knowledge. I've added a bunch, and have started using it as kind of a scratch pad. Say I run across a NPC which starts a quest, if they're not in the wiki, I may jot a note down about it in the /note window as a reminder that I need to work on this quest, then flip over and add more detailed info to the wiki. As I find more info about the quest, I fill in the blanks in the wiki as I go.
If you want to add anything, please read the first page and add a link to whatever you add in the appropriate category to keep things nice and tidy.