Page Types represent the data object of each page. Page Types often contain special backend functionality that makes them unique beyond standard pages. Page Types can have different page templates assigned to them for output. This helps to decouple custom page functionality from page design.
Included in each theme are several Page Templates. Page Templates can be thought of as re-usable visual formats for the various kinds of pages on your site. For instance, you might have a "Left Sidebar" page on your site that features a left-aligned sidebar design. You might also have a Page Template called "Right Sidebar, "Homepage", or "Full Width". Each takes the same blocks you've placed on your page and displays them differently. Like themes, these can be swapped out on-the-fly to change how your page looks with just a few clicks.