The Acquia company exists in the best for-profit tradition of the Open Source business model, which is that of providing services paid for by clients built on an Open Source software system — one, that is, published under an Open Source license and available free of charge.
Dries Buytaert, the creator and founder of the Open Source Drupal Community and the founder of the Acquia company, often compares Acquia to Red Hat, as Drupal is to Linux. The rationale here is that Red Hat is a distribution of the freely available Linux operating system, but is not itself freely available since its distribution adds a commercial service in and of itself. And clients have shown that they are willing to pay for that service.
However, I would suggest that the metaphor model might be closer to the following: Acquia is to Drupal as Canonical (the commercial sponsor of Ubuntu) is to Debian, because Acquia Drupal is still provided free of charge under the GPL license, as is the regular vanilla Drupal tarball distribution you can download from the Drupal site itself, which you have been working with in this book. What Acquia provides are the following services:
Packaging via software integration for the Drupal core and third-party modules
Integration of the Drupal core with selected third-party modules, based on best practices and tested procedures
Integration with Mollom, an anti-spam service
Monitoring via the Acquia Network
First, Drupal ...