Categories help to find a topic on your website. Also, you can Categorise your content. If you write article about more than one Topic. Ex: Technology, Food, Health Care. If any visitors need to see only about Technology directly, they can click Technology and view the ony Technology articles. Likewise, you should divide your articles to make user-friendly and helps for SEO, If you use Related plugin this plugin show the similar items by using Categories. Tags are like keywords. Visitors easily find your content by using Tags. It’s also helps to find the relevant content on your blog. Each post you should give not more that 5 tags. If you give 10 or 20 tags on each post it’s become spammy.