Yeah, it's cool, but what does it mean? What do the nodes and lines represent?

What do the colors mean? blue: for links (the A tag) red: for tables (TABLE, TR and TD tags) green: for the DIV tag violet: for images (the IMG tag) yellow: for forms (FORM, INPUT, TEXTAREA, SELECT and OPTION tags) orange: for linebreaks and blockquotes (BR, P, and BLOCKQUOTE tags) black: the HTML tag, the root node gray: all other tags