See the documentation:
Categories
pMachine lets you create a list of categories (Rock, Jazz, Blues, Metal, etc.). Then, every time you create a weblog entry you can assign it to one of your categories. Visitors of your site can then view entries from a specific category.
Categories are defined in your control panel by clicking the Categories menu. You can "add", "edit", and "delete" categories from this page.
Categories can ether be global or weblog-independent depending on the preference you set in the category page. With global categories you can define one master list of categories and all of your weblogs will share it, while Independent categories allow each weblog to have an independent list.
