Posts, categories, and pages are 3 different things.
Posts are made to your home page: they are all the articles you write from day to day. They appear on a dynamically changing "page" which is the main part of your blog. Your home/blog page is not a page you can actually edit. It changes whenever you write a post.
Pages are static and don't change. You use them for basic information such as a page "about" you. Background information, or things your readers should know about your topic.
Categories are applied to your posts/articles. Like one post could be about sports and you'd make a category for "sports" and another post could be about cooking and you'd make a category for cooking. Then you write another article about sports. Well, you'd assign that post to the category "sports."
Categories will generate "pages" (which you also cannot edit) with all the posts you have in each category. Your readers can choose your "sports" category to see everything you have written about that.
There is a wealth of information in the support pages here: