From the JDOM FAQ:
What is JDOM?
JDOM is, quite simply, a Java representation of an XML document. JDOM provides a way to represent that document for easy and efficient reading, manipulation, and writing. It has a straightforward API, is a lightweight and fast, and is optimized for the Java programmer. It's an alternative to DOM and SAX, although it integrates well with both DOM and SAX.
What is JDOM not?
JDOM is not a wrapper for the W3C's DOM, or another version of DOM. JDOM is a Java-based "document object model" for XML files. JDOM serves the same purpose as DOM, but is easier to use.
JDOM is not an XML parser, like Xerces or Crimson. It is a document object model that uses XML parsers to build documents. JDOM's SAXBuilder class for example uses the SAX events generated by an XML parser to build a JDOM tree. The default XML parser used by JDOM is the JAXP-selected parser, but JDOM can use nearly any parser.