DOM Definition

MDN says "The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the web." The DOM is language neutral which means it can be implemented in any programming language. The DOM represents structural pieces of a web page as objects so programs can change the structure, style, and content. The DOM is what enables a webpage to be dynamic. The DOM represents a document in a tree structure. Every branch of the tree ends in a node, and every node contains objects. The DOM then provides premade methods that can be used to manipulate the objects represented in the DOM. According to W3c the DOM "defines the logical structure of documents and the way a document is accessed and manipulated.

DOM Specifications

The DOM was first published in 1998, and according to Wikipedia DOM level 4 is the latest version it was published in 2015 and is fully supported on all popular browsers.

DOM Implementation in Browsers

From what I can gather the server serves up a platter of HTML to the browser. This platter resembles a pile of raw materials. The DOM provides the specifications that the browser uses to create objects from the HTML elements that make up a webpage. Along with those specifications the DOM also defines the functions that are used to manipulate webpages.

Summary

The DOM is what makes the Web go around. It's what makes html dynamic.