From wordpress.com:
Posts
Posts are entries listed in reverse chronological order. You can control how many posts are displayed at a time in Settings -> Reading.
More information about posts can be found in the WordPress support site.
https://en.support.wordpress.com/posts/
Pages
Pages are static and are not listed by date. Pages do not use tags or categories. An About page is the classic example.
More information about pages can be found in the WordPress support site.
https://en.support.wordpress.com/pages/