New to the WordPress ecosystem are Block-Based Themes. With the advent of the new Gutenberg editor is the need to have themes built from the ground up to take advantage of building with blocks and give users a starting point for a well-designed site.
Block-Based Themes are built entirely with templates composed of blocks and allow editing for all of those blocks in the editor window. In its intended form, a Block-Based Theme will have template parts used for Block Areas like the navigation, the sidebar and the footer.
Block Templates are made up of a list of blocks. Any WordPress block can be used in a template, and provides a scaffolding of predetermined content areas for users to enter information in the editor. Block Templates can be selected from the Inspector under Document > Post Attribute > Template > Template Name or as a Custom Post Type that renders a list of blocks for a post type like a Product Review.
Presently, a full Block-Based Theme can only be built and used under the Full Site Editing experimental flag in the Gutenberg plugin. But theme authors can begin the first step of creating templates for content areas that can be called as a template in the Inspector under Document > Post Attribute > Template > Template Name or by building a Custom Post Type that can be used in current themes.