Both HTML and XML are hierarchical tree structures of tags (or elements). The window contains a document; documents contain an HTML element, which in turn contains a child header and body element, and so forth. The best definition of the DOM is the one specified by the W3C:
“The Document Object Model is a platform- and language-neutral interface that will allow programs and scripts to dynamically access and update the content, structure and style of documents. The document can be further processed and the results of that processing can be incorporated back into the presented page.”
Most of the cross-browser concerns pertaining to variations in the DOM are silently taken care of for you in jQuery. You don’t need to worry (much) about the differences in the DOM redundant between the Gecko-friendly browsers, IE, Safari, and Opera.
Source: Otero Cesar, Rob Larsen (2012), Professional jQuery, John Wiley & Sons, Inc