You can add tags to categorize your posts, which displays Categories on your sidebar. Categorizing a post in this way allows visitors to your blog to easily find posts that cover a specific topic.
Adding Categories to a Post is as easy as opening the Post Options, and using the Categories section to select any previously added Categories that apply, and adding any new Categories that seem appropriate.
Once you add a Category it's always available to use for future posts.
Any Categories that are used with a blog post will show up in the Categories section of your blog's sidebar. Clicking on a Category will display all blog posts that have been tagged with that Category.
If you don't see a Categories section on your sidebar, you can add one by dragging a Categories element to the page. Note: the Blog Sidebar elements are only available when you're working on a blog page in the editor.
We recommend you use a certain amount of restraint when creating Categories. If you keep the number to a minimum (say, less than twenty or so) it'll be easier for visitors to find posts that interest them. Creating a new Category for almost every post makes it difficult to navigate and significantly less useful for your visitors.