Window object: This is the topmost object in the hierarchy. It represent the content area of browser window that consists of HTML documents. Each frame is also a window that has some actions inside it.
Document object: This object gets loaded in a window and consists of objects of different kind in the model. It consists of the content that will be written in the script.
Form object: Form objects are used for more interaction with the users. It represents the form elements inside <FORM>...</FORM> tag.