XPDL is the Serialization Format for BPMN
BPMN is a visual process notation standard from the OMG, endorsed by WfMC, and broadly adopted across the industry. But the BPMN standard defines only the look of how the process definition is displayed on the screen. How you store and interchange those process definitions is outside the scope of the standard, and this is where XPDL comes in. XPDL provides a file format that supports every aspect of the BPMN process definition notation including graphical descriptions of the diagram, as well as executable properties used at run time. With XPDL, a product can write out a process definition with full fidelity, and another product can read it in and reproduce the same diagram that was sent.

XPDL Enables a Process Definition Ecosystem
XPDL is used today by more than 80 different products today to exchange process definitions. As a greater percentage part of the organization starts to use process tools for everyday work, it will become less and less reasonable to take a single-vendor strategy to process definition work. Users need to go beyond the vendor lock-in, and take a "best of breed" approach that allows the use of their favorite process technology to accomplish specific process oriented tasks, such as simulation and optimization, and future tasks that we can only dream of today.

XPDL is extensible so that it allows each different tool to store implementation specific information within the XPDL, and have those values preserved even when manipulated by tools that do not understand those extensions. This is the only way to provide for a "round trip" through multiple tool and still be able to return to the original tool with complete fidelity.

How Does XPDL Compare to BPEL?
BPEL and XPDL are entirely different yet complimentary standards. BPEL is an "execution language" designed to provide a definition of web services orchestration. It defines only the executable aspects of a process, when that process is dealing exclusively with web services and XML data. BPEL does not define the graphical diagram, human oriented processes, subprocess, and many other aspects of a modern business process: it simply was never defined to carry the business process diagram from design tool to design tool.

XPDL Conformance