DOM is the Document Object Model. It is a way forward.
It has become a standard, defined by the w3c. The current, complete standard is DOM version 1 while version 2 has already been put forward by the w3c as a recommendation (pending approval as a standard) and version 3 is being drafted.
All of the major graphical browsers have implemented DOM as an integral part (part-and-parcel) of their products. DOM was implemented in MSIE5.5 to a considerable extent. It has been implemented more-so in MSIE6 as well as Net6 and Opera7.