The DOM is the Document Object Model. The DOM represents a document on the web as nodes and objects so that programs can interact with the page. This allows scripting languages, like JavaScript, to modify the page using specific methods and properties. For example, developers can use the "querySelectorAll" method to get a list of all elements on the page that meet a given criteria.
The W3C is responsible for standardizing and creating the specifications for the DOM. It was necessary to have one organization hold this responsibility because in the early days of the web, developers would have to code different web pages for different browsers. When the W3C created their DOM standard, all commonly used browsers adopted it, meaning developers only had to use this single API to interact with web pages using a scripting language.
When a browser renders an HTML page, it parses the document into objects, or nodes. Thus, the developer is to use a scripting language to interact with the objects on a page. The DOM is visible in the developer section of the browser that appears when "inspect element" is clicked on a page. Each browser creates this model according to the DOM standard so developers can create the same web page for all browsers.
Overall, the DOM is a very useful API that is fundamental for allowing JavaScript to work on a modern web page. Understanding the different methods and properties of the DOM will allow developers to use JavaScript (or another scripting language) to interact with an HTML document in a browser.