DOM (for Document Object Model) is a way of conceptualizing the contents of a web page. You can think of this as an agreed-upon way of describing elements in the document, just as there is an agreed-upon way of describing houses on your street in English. We all acknowledge, for instance, that the long strip of tarmac dividing houses on one side from the other is called the “road”. Each house is numbered, but we might also address them by position: “The third house on the left”, for example, or “The house after the red house”. While each house or building will be entered in a slightly different way, we call that entry by the same name: “door”.
In 1998, the W3C released DOM Level 1, a standardized API for referring to the content of web pages, and modern graphical browsers now support it to a high degree. The DOM has now become so ubiquitous that the scripting we are about to look into has been given yet another term: DOM Scripting.