A big difference between Slate and other rich-text editors is that Slate is built on top of a nested, recursive document model—much like the DOM itself. This means you can build complex components like tables or nested block quotes, just like you can with the regular DOM.
Each Slate document is made up of Document, Block, Inline and Text nodes—again, very similar to the DOM.
The top-level node of a Slate document is the Document node. The Document then has child Block node children.
After that, nesting can occur to any depth. Block nodes can contain other Block nodes, or they can contain Inline or Text nodes. And Inline nodes can contain other Inline nodes or simply Text nodes.
Each Document, Block and Inline node implements a Node interface, to make working with the nested tree easier.
As the leaves of the tree, Text nodes contain both the text content of the document as well as all of the text-level formatting, like bold and italic. In Slate, that formatting is referred to as Marks, and Marks are applied to individual Characters.
Just like the DOM, Slate's Block and Inline nodes can be "void" nodes, meaning that they have no content inside of them. When the isVoid flag of a node is enabled, it will be specially rendered, to preserve the editing experience that a user expects, without any extra work.
Note that even though the node is "void", it will still have an empty text node inside of it. Because of...
One constraint of Slate documents is that the leaf nodes are always Text nodes. No Block or Inline node will ever have no children. It will always have at least an empty text node. (However, you can render text-less nodes, see the Void Nodes section above!)
This constraint means that Selections can always refer to text nodes, and many text-node-level operations are always "safe" regardless of the tree's structure.