For those not familiar with agile or extreme project management (the terms are often used interchangeably), Answers.com probably offers the most concise description:
"Extreme project management differs from traditional project management mainlyTo learn more, the Agile Project Leadership Network (APLN) offers 8 core principles of Agile Project Management, along with several links and resources.
in its open, elastic and undeterministic approach. The main focus of XPM is on the human side of project management (e.g. managing project stakeholders), rather than on intricate scheduling techniques and heavy formalism."
The Cutter Consortium also offers a wealth of information on Agile Project Management.