What is the DOM and how does JavaScript interact with it?

When the browser reads your HTML file, it builds a family tree from it — parents, children, and siblings. This tree is called the DOM. JavaScript can walk this tree, find any member of the family, change their properties, add new members, or remove them. Every time you change a node, the browser has to redraw part of the page — so it's like repainting a room every time you move a piece of furniture.