Process definition files are usually written in an XML format. These files can easily be stored on a file system during development. However, whenever you want to make your knowledge accessible to one or more engines in production, we recommend using a knowledge repository that (logically) centralizes your knowledge in one or more knowledge repositories.
Guvnor is a sub-project that provides exactly that. It consists of a repository for storing different kinds of Knowledge, not only process definitions but also rules, object models, etc. It allows easy retrieval of this knowledge using WebDAV or by employing a Knowledge Agent that automatically downloads the information from Guvnor when creating a Knowledge Base, and provides a web application that allows business users to view and possibly update the information in the Knowledge Repository. Check out the Drools Guvnor documentation for more information on how to do this.