The DOM is a tree based mechanism that allows u to load or 
create an XML document,gather errors if any: access and 
manipulate the information and structures contained within 
the document and save the document back out to an XML file 
if necessary. Construcing the DOM requires reading the 
entire XML structure and holding the object tree in 
memory,so it is much more CPU and memory intensive.
    the SAX is an event-driven ,serial-access mechanism for 
accesing XML documents. SAX is the fastest and least memory 
intensive mechanism that is currently available for dealing 
with XML documents.