Categories
Categories and tags are used to make it easier for your users to navigate your website. Used correctly, they can increase your website’s overall usability.
Each post must have one Category, but it’s up to you whether you have Tags or not.
Some good advice is that your Categories are like your site’s table of contents, and your Tags are for your site’s index.
Tags
Tags are a non-hierarchical taxonomy. This means that there are no parents and children, but the information is presented as flat. A post in the Outdoor Category might have the tags plants,animals , water, and knowledge.