Moodle currently has two popular implementations for web services: Remote-Learner Web Services library, and Moodle Networking. Remote-Learner Web Services are SOAP-based and Moodle Networking is XML-RPC-based. The SOAP library allows remote manipulation and the creation of courses, users, user enrollments, grade passing, and SSO. It is also a great example of what you might accomplish by leveraging the techniques covered in this book. The XML-RPC library provides a secure channel, SSO, and full Moodle API exposure (with serious caveats) to the trusted site.
During the writing of this book, we had to make what some may consider a controversial decision to focus more pages on the RL Web Services library than on Moodle Networking. Moodle Networking is the official core Moodle method for web services. So this would be the argument in favor of its importance. Or even that it is more important than the RL WS library. However, in practice...