OpenACS is an Open Source Web Community System. A web community is where while the users are responsible for generating most of the content. Sites like Slashdot, Yahoo!, and Photo.net are examples of community sites. They usually consist of features like a bulletin board (bboard, sometimes called a discussion forum), news, and chat type features.

It is Open Source for several reasons, some ideological, some practical, and some historic. In the software industry, and indeed in the web world, software licensing only generates a small part of the total revenue. Even large organizations like ORACLE and SAP make most of their money from training and services. In order to make what money they do from licensing they have enormous sales and marketing forces. A small organization like arsDigita (the company which wrote the original version of the ACS -- before it was OpenACS) was planned to focus mostly on services and wouldn't have wanted the hassle of licenses to begin with. Making their product Open Source also gave them free advertising with the technically savvy, it also allowed them to get free QA work, software patches, packages, and feedback. All of these are vital to the success of any software package.