Note: If you haven’t done your basic site configuration yet, check out this post to begin with that.
Posts are the typical content you see on blogs (hence the term blog post—see, this isn’t that hard). They’re organized by date, author, categories, and tags, and they often allow comments and social links. Posts will be associated with a particular page—usually people think of this as their blog page, and it will be titled something really obvious like Blog. You can only have one posts page on a site.
Pages are typically for permanent information, such as About pages, or Contact pages. Pages can be organized in parent/child relationships; for example, you could have an About page with several child pages, one for each of the contributors for a site.
Pages don’t show up in the blogstream (that is, the posts), you must add them to a menu. Child pages will show up in a dropdown menu (submenu) from the parent page’s menu link.