The two most common low-level APIs today are SAX, the Simple API for
XML, and DOM, the Document Object Model. Additionally, JDOM (which is
not an acronym, nor is it an extension of DOM) has gained a lot of
momentum lately. All three of these are in some form of
standardization (SAX as a de facto, DOM by the W3C, and JDOM by Sun),
and are good bets to be long-lasting technologies. All three offer
you access to an XML document, in differing forms, and let you do
pretty much anything you want with the document. I'll spend
quite a bit of time on these APIs, as they are the basis for
everything else you'll do in XML. I've also devoted a
chapter to JAXP, Sun's Java API for XML Processing, which
provides a thin abstraction layer over SAX and DOM.