Post
If you’re going to have a blog, you’ll mostly be working in the Posts area of your WordPress dashboard. Posts are your actual blog posts. View them as articles in a newspaper. When you publish a new one, the last one gets pushed down and the new one is at the top and the most recent.
Posts can also be filed into Categories and Tags to help sort through your blog posts. After many years of blogging you’ll likely have hundreds of posts so having an organized filed system in place will make finding older posts easier when you file them within categories and tags. For more information about the difference between a Category and a Tag, see this.
Page
A page is static information. For example, your About page is a page you create once and it remains. It’s not filed within your blog posts but remains static unless you go back and edit the page in the future.