Post categories
The Blog Plugin supports nested and multiple categories. "Nested" means, that you can have subcategories (for example the category "Operating Systems" with the subcategories "Linux" and "OS X"). "Multiple" means, that you can assigne one post to more than one category.
You can edit the categories via "Quicklinks" -> "Post Categories". Make sure there's exactly one "Root" category, which doesn't havea "Parent-Category". Just don't delete the existing one, and nothing can go wrong. All other categories need a "Parent-Category".