Ever wondered why you need to use both on your blog? Let me tell you…
Basically, Categories are meant for a wide grouping of your posts. These are general topics that you blog about. You can use categories to identify what your blog is about. It assists readers to find the right sort of content on your site. You must categorize your posts.
Now Tags are actually meant to describe specific details of your posts. These are basically your site’s index words. They are the micro-data that you can use to micro-categorize your content. Tags are not hierarchical & you cannot sub-categorize them. Using tags in your posts is optional.
Hope this cleared things up a bit, folks!