A page design in SXA is a group of partial layouts that make up the design of a web page.
In SXA, you build your website with reusable pieces of content and layout. All these pieces together create the layout for your pages.
A page design in SXA is a selection of partial designs that help you to structure your pages. You can, for example, make sure the headers and footers are always in the same place. You can also create a page design to set up a page structure for specific pages, such as a blog page, a landing page, a product page, and so on. Content authors can then place content in these preset layouts.
You can also use rules to select a page design.
You assign a page design to a page (or pages of a specific type) to define the elements and renderings that you want to appear. For example, a blog page may need a header with a navigation component, a main placeholder for the content (post list, categories, archive, tag cloud, recent comments), and a footer with company information.
Data templates are the schema for Sitecore content. Any content item in a Sitecore database is based on a data template.
In SXA, you can link page designs to data templates. In this way, you can link your content types to your page layouts and keep the layout of your site consistent. It is very convenient to assign a page design to the template for web pages that you know you will be using a lot, so that they look consistent.