Like all WordPress based sites, Wikity has both "categories" and "tags". On Wikity we use tags to make things findable and categories to collect resources associated with a particular project.
What's a project? It's any sustained investigation for which you plan to create a significant body of linked work.
Mostly, it's things you work on with other people, where you've defined a community goal. So Kate Bowles and Rolin Moe put together a study group on innovation which categorizes their work as Innovation criticism. If you were working with a class on the politics of water, you might have your class categorize their work "Politics of water".
You might also categorize your own solo projects this way. If you are working on creating resources for a workshop or a class, or if you just want to do your own sustained investigation of a subject, a category can help others find your body of work and invite others to join your effort.
Because categories will be used in this way we ask that you create them sparingly. Don't use them as simple descriptors of content. Before you apply a category, ask yourself "Is this a description of the content of the post, or an indicator of the project it was created for?"
If it's the former, it's a tag. If it's the latter it's a category. Keeping to this norm will help people on the site easily identify projects they might enjoy joining, leading to a better overall Wikity experience.
One other note -- to make the menu on the bottom of the front page look consistent we ask that you write category names out with spaces, capitalizing the first letter of the category phrase. Again, "Innovation criticism" and "Politics of water" provide the model here.