Categories provide a helpful way to group related posts together, and to quickly tell readers what a post is about. Categories also make it easier for people to find your content. Categories are similar to, but broader than, tags.
Depending on the theme and widgets you have activated, categories can be displayed at the top or bottom of posts, on the individual post view, and/or in the sidebar. You can assign multiple categories per post (you must assign at least one), and categories can be organized hierarchically.