Key Concepts 
Edit
Blocks Blocks
Blocks are an abstract unit for structuring and interacting with content. When composed together they create the content for a webpage. Everything from a paragraph, to a video, to the site title is represented as a block.
Blocks come in many different forms but also provide a consistent interface. They can be inserted, moved, reordered, copied, duplicated, transformed, deleted, dragged, and combined. Blocks can also be reused, allowing them to be shared across posts and post types and/or used multiple times in the same post. If it helps, you can think of blocks as a more graceful shortcode, with rich formatting tools for users to compose content.
The settings and content of a block can be customized in three main places: the block canvas, the block toolbar, and the block inspector.
Composability Composability
Blocks are meant to be combined in different ways. Blocks are hierarchical in that a block can be nested within another block. Nested blocks and its container are also called children and parent respectively. For example, a Columns block can be the parent block to multiple child blocks in each of its columns. The API that governs child block usage is named InnerBlocks.
Data & Attributes Data & Attributes
Blocks understand content as attributes and are serializable to HTML. To this point, there is a new Block Grammar. Distilled, the block grammar is an HTML comment, either a self-closing tag or with a beginning tag and ending tag. In the main tag, depending on the block type and user customizations, there can be a JSON object. This raw form of the block is referred to as serialized.
<!-- wp:paragraph {"key": "value"} -->
<p>Welcome to the world of blocks.</p>
<!-- /wp:paragraph -->
Blocks can be static or dynamic. Static blocks contain rendered content and an object of Attributes used to re-render based on changes. Dynamic blocks require server-side data and rendering while the post content is being generated (rendering).
Each block contains Attributes or configuration settings, which can be sourced from raw HTML in the content via meta or other customizable origins.
Transformations Transformations
Blocks have the ability to be transformed into other block types. This allows basic operations like converting a paragraph into a heading, but also more intricate ones like multiple images becoming a gallery. Transformations work for single blocks and for multi-block selections. Internal block variations are also possible transformation targets.
Variations Variations
Given a block type, a block variation is a predefined set of its initial attributes. This API allows creating a single block from which multiple configurations are possible. Variations provide different possible interfaces, including showing up as entirely new blocks in the library, or as presets when inserting a new block. Read the API documentation for more details.
More on Blocks
Reusable Blocks Reusable Blocks
A reusable blocks is a block (or multiple blocks) that can be inserted and edited globally at once. If a reusable block is edited in one place, those changes are reflected across all posts and pages that that block is used. Examples of reusable blocks include a block consisting of a heading whose content and a custom color that would be appear on multiple pages of the site and sidebar widgets that would appear on every page.
Any edits to a reusable block will appear on every other use of that block, saving time from having to make the same edit on different posts.
In technical details, reusable blocks are stored as a hidden post type (wp_block) and are dynamic blocks that “ref” or reference the post_id and return the post_content for that block.
Patterns Patterns
A block pattern is a group of blocks that have been combined together creating a design pattern. These design patterns provide a starting point for building more advanced pages and layouts quickly. A block pattern can be as small as a single block or as large as a full page of content. Unlike reusable blocks, once a pattern is inserted it doesn’t remain in sync with the original content as the blocks contained are meant to be edited and customized by the user. Underneath the surface, patterns are just regular blocks composed together. Themes can register patterns to offer users quick starting points with a design language familiar to that theme’s aesthetics.
Templates (in progress) Templates (in progress)
While the post editor concentrates on the content of a post, the template editor allows declaring and editing an entire site using blocks, from header to footer. To support these efforts there’s a collection of blocks that interact with different parts of a site (like the site title, description, logo, navigation, etc) as well as semantic areas like header, sidebar, and footer. Templates are broken down between templates (that describe a full page) and template parts (that describe reusable areas within a template). These templates and template parts can be composed together and registered by a theme. They are also entirely editable by users using the block editor. Customized templates are saved in a wp_template post type. Block templates include both static pages and dynamic ones, like archives, singular, home, 404, etc.
Note: custom post types can also be initialized with a starting post_content template that should not be confused with the theme template system described above.
Global Styles (in progress) Global Styles (in progress)
Describes a set of configuration and default properties of blocks and their visual aspects. Global Styles is both an interface (which users access through the site editor) and a configuration system done through a theme.json file. This file absorbs most of the configuration aspects usually scattered through various add_theme_support calls to simplify communicating with the editor. It thus aims to improve declaring what settings should be enabled, what attributes are supported, what specific tools a theme offers (like a custom color palette), the available design tools present both globally and on each block, and an infrastructure that allows to enqueue only the relevant CSS based on what blocks are used on a page.