This is our top-level category, or looking at it another way, it is the root category. Ideally, every other category should be a subcategory of at least one other, and every article should be in at least one category; many will fit well into two or more(e.g. location and subject-matter).
The "Content" category is intended for all articles about the subject matter of the wiki, whilst "Organisation" is intended for all administrative and organised matters behind the wiki. A full list of existing categories can be found at Special:Categories.
To add a new category to a page, just add
[[Category:<Categoryname>]] to the end of the article or existing category page.