A list of terms into which all the content in your site can be sorted. Each post must have a Category.
Tags are also terms that can be applied to your content, but they are optional and it is common to apply more than one tag to a Post.
Categories vs Tags
You should have a Category for all the main divisions of your site. Each Post should fit into a single Category, or possibly two. (If your Posts routinely fit into more than one category, rethink your categories). If you have a recipe site, your categories might be Appetizers, Entrees, Salads, Side Dishes, Soups, Desserts and perhaps another one on Kitchen Tools.
Tags are more casual although it helps to think a little about what main tags you might want to use. For that recipe site, tags might include: vegan, chocolate, gluten-free, and other minor terms that your users may want to use to find posts, but which don’t indicate major site divisions. Each post can have multiple tags. If a certain tag could be applied to EVERY Post, you don’t need it. For instance it would be silly to have the tag “food” in a recipe site.
Another good use of tags is for audience: if some of your posts are appropriate for students and others for faculty, you can tag them this way. If some are appropriate for both, tag them students, faculty and searching for either tag will get this post. Remember that “student” is different from “students” and decide ahead of time which version you will use for the major tags you want to provide.
A page in WordPress is usually used for About Us and other “static” content. A page is a unique item and it ties readily to the Menu. We often speak of the main Pages tied to the menu as “landing pages”.
A Post is a piece of content – an article – with a Category. Each entry in a typical blog is a Post.
Because search is so important to a modern site, it is important that the Post has an excellent title that completely explains what the post is about. Often folks will decide whether to read your Post based on the title and first few lines. For example, you could name an article “Parking” but if it is actually an article about how students can park for evening events it would be better to call it “Evening Parking for Students” and then also tag it “students”. Your first line should precisely detail what the Post is about.