I find the differences between a WordPress page and post are as follows:
A page is static, one off such as a contact page or about page. This page will really get changed and you ideally will have a link to it in your header so that it is indeed visible on all of your websites posts.
A post is the main content on a blog, posts are highly variable, can be updated and can get listed and sorted depending on their tags and categories. Users can be notified of your posts and engage in discussion on your posts also.