According to Philip Greenspun, the main creator of the ACS:
"The ArsDigita Community System it is a toolkit of software that will help you build Web services with a collaborative dimension, ranging from knowledge management within companies to B2C ecommerce to product support and community among the customers. The software is free and open-source and has been tested in heavy use since 1995. "
"The software and underlying philosophy are documented in a full-length textbook for Web developers (also free and available online at http://photo.net/wtr/thebook)" ( Using The ACS). We highly recommend you to read this book. It will save you lots of trial-and-error, time, grief and will help you understand how to build a great web service. Chapter 3 is of special interest, because that's where the data model and ideas of the ACS are explained, but the whole book is great.
If you are serious about building a web service, you should also read ArsDigita Server Architecture, a way of building and delivering reliable web services.
The port for the PostgreSQL RDBMS was started and coordinated by Ben Adida (who also helped to write the original ACS) and Don Baccus, and is maintained as a community project.
The combination of GNU/Linux, AOLserver, PostgreSQL and OpenACS enables you to build very sophisticated web sites with completely free software. The development and demo web site for OpenACS is http://openacs.org.