This is our top-level category. (Looking at it another way, it is our root category!)
Ideally, every other category should be a subcategory of at least one other category, and every article should be in at least one category; many will fit well into two (eg location and subject-matter). A full list of existing categories can be found at Special:Categories.
New categories are best created from below: just add [[category:categoryname]] to the article or category (after checking whether any existing category is suitable).
Discussion of principles of categorisation/categorization may be found at project:categories.