DOM (Document Object Model), contrary to what the name implies, is not a model. Literally DOM no, it is not a model, but figuratively we could say that it is a model of access to web data visualized as objects. It is a programming interface that provides a standard series of objects to represent HTML, XHTML, and XML documents. It provides a structured form of the document and defines the way in which programs can access it in order to vary structure, style and content.
When we use HTML + CSS we create static pages since they are not personalized enough, but if we add JavaScript we can customize them and turn them into dynamic pages. DOM was created as a specification to approve Java and JavaScript scripts to be compatible between web browsers. It is the most influential innovation in the development of the most complicated web applications and dynamic web pages.
What DOM does is transform XHTML documents into a group of elements called nodes. These nodes are interconnected and represent the contents of web pages and the relationships between them. It is called a node tree because of the appearance they show when all the nodes are joined.
You don’t need to do anything concrete to start using the DOM. Different browsers have DOM guidelines and these guidelines have varying degrees of conformity to the current DOM standard. But what they all have in common is that they use the document object model to make web pages viable to script.