A process archive is a zip file containing information about a process graph.
The central file in the process archive is
processdefinition.xmlwhich contain the structure of the process graph. The
processdefinition.xml also contains information about actions and tasks. A process archive can also contain other process related files such as classes, ui-forms for tasks.
Here's a sample process.par file structure.
You can deploy the process archive just any other archive (by default on the deploy folder of JBoss server). Once you've deployed it, you can start work on your process, creating new Process Instances.
Here's the sample HelloWorld .par file generated by JBoss JBPM plugin: