In the ion platform, you publish pages & experiences using templates (called "Master Pages" in ion). ion comes with a set of templates that are custom-branded based on your look & feel. You can also make your own templates using HTML, or have the ion team create custom templates for you as a special project.
The benefit of templates is marketer freedom. A designer can create a page layout once and then the marketer can use it, over and over again, changing it, updating it whenever they need to—creating hundreds, or even thousands, of versions of the page without ever having to use technical or design resources.
This graphic helps demonstrate the relationship between ion Frameworks, Templates, Themes and Creatives.