Master pages represent the powerful concept of sharing the same header and footer for all pages on the website. It allows you to manage repeated items, such as the site logo, main menu and footer content in a single place.
The root of the content tree is always a master page. You can also configure any other page template to be a master page by enabling the Master page option on the General tab of the page template editing interface at Site Manager -> Development -> Page templates.
The following figure shows how the same master page is used for the home page and product page. As you can see, the pages are inserted inside the master page: