Explaining the DOM
The document object model can be compared to a tree, with branches and leaves. Elements are similar to branches, which can contain even more branches. Nodes are leaves, and contain objects. The DOM allows users to view, alter, or style the document on HTML or XML code.
DOM Specifications
Each browser is responsible for loading HTML documents into the DOM, but the W3C maintains the standard DOM. WHATWG develops the standard, and the W3C publishes stable builds from WHATWG.
DOM Implementation in Browsers
Most main browser vendors use nearly the same DOM. I looked at a video to view the Apple DOM, and it looks different from what Google Chrome and Microsoft Edge use. I also looked at Internet Explorer, but it hasn't been updated of course.
Summary
The DOM shows HTML and XML documents in a tree structure to help the user alter code and debug. The W3C maintains the standard DOM, but WHATWG develops the new standards that W3C publishes.