Posts are entries listed in reverse chronological order on the blog home page or on the posts page if you have set one in Settings → Reading. If you have created any sticky posts, those will appear before the other posts. Posts can be found in the Archives, Categories, Recent Posts, and other widgets. Posts are also displayed in the RSS feed of the blog.
Pages are static and are not listed by date. Pages do not use tags or categories. An About page is the classic example. Pages can be displayed in the sidebar using the Pages widget.