What is the difference between using categories and tags?

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You can use categories to group content so that blog readers can browse the blog for a specific topic. Also, categories provide categorization information to search engines and content aggregators. You can create a hierarchy of categories up to nine levels deep; however, it is usually sufficient to use only one level for personal blogs. You must have the admin, webadmin, or webcompose role for a blog application to create categories.

Tags are used by website search engines to group blog content so that information can be found by readers searching for content about a particular subject. You can create tags when you create a post or a comment document. However If you do not specify any tags, Content categories are used by default.