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Search engine and WordPress.com treat tags in exactly the same way but it is important to note that the Categories we assign to posts are for defining broad subject areas and Tags are used to describe your post in more detail.
Envision yourself in an old style library and remember how the catalogs we used were based on keywords Categories and keyword Tags. No move ahead and envision yourself logged onto the internet and using a search engine. Which keyword Categories and which keyword Tags would you use to locate your posts?
Successful blogs draw between 30 - 60 % of their incoming targeted readers. from search engine referrals. Targeted readers are those who arrive on your site because they already have an interest in the content your posts and blog contain. They have typed keywords and phrases into search engines and located your posts in the SERPS (search engine page results) and clicked through to the posts. The fact that targeted readers already have an interest in the content in your posts means they are likely to read more than one post, more likely to leave comments, and more likely to become regular readers/subscribers.
Consequently, the first step for any blogger to take to establish a flow of targeted readers to their blog is learning and applying the use of basic SEO elements so their posts and blog are actually accessible to potential readers, who use search engines. Part of those SEO techniques are the selection of a minimal number of Categories and Tags we assign to posts.
I don't know what you mean by the same Categories and Tags not being used over and over again. If the current post in front of you is also in the same Category as your other posts (example your blog is on Marine Mammals) then use both subcategories (whales, walrus, porpoises, seals, sea lions, etc.) and Tags (North Atlantic, Pacific, diseases, food, migration) to home in on describing specific post content.
Categories are like the chapter titles on a Table of Contents of a book. Tags are like the index terms that are found in the back pages of a book. Both are based on key word descriptions that naturally occur in any written language.
Whenever you get confused about categories and tags imagine they are in a library looking for a book on a specific topic. Then, I ask yourself this: Which key words would I use to find book on that topic in a library catalog?
If you change the word “book” to “blog” you will know how to find a blog on any specific topic by typing keywords into a search engine and getting back the results. If you change the word “blog” to “post” and imagine which descriptive key word terms a potential reader might use to look for a post like yours that may help you assign categories and tags more effectively.