Pages are static content that are not time dependent. Normally, your website’s “static” content is used on Pages, like the About, Services, Contact etc. pages.
Posts are a dynamic content type. They have a timestamp and will be listed on your website in reverse chronological order. Each post is assigned to a Category. You can also Tag your posts. Posts are displayed in an RSS Feed.
Blogs use Posts as they can be organized in a manner so the reader easily can find a post about a specific subject or from a specific time.