The Simple API for XML defines the events and interfaces used to interact with a SAX-compliant XML parser. You can find the complete SAX specification at www.saxproject.org.
The JDOM project was created by Jason Hunter and Brett McLaughlin and lives at jdom.org/. At the JDOM site, you can find code, sample programs, and other tools to help you get started. (For developerWorks articles on JDOM, see Resources.)
One significant point about SAX and JDOM is that both of them came from the XML developer community, not a standards body. Their wide acceptance is a tribute to the active participation of XML developers worldwide.
You can find out everything there is to know about JAXP at java.sun.com/xml/jaxp/.