There are a number of sources of information on the ArsDigita Community System (ACS). I recommend you start with Using the ArsDigita Community System. This is a more concise intro than Chapter 3 of Philip and Alex's Guide to Web Publishing, which is probably the next thing you'll want to read. In addition, it has links to finding the software, a FAQ, and some example uses.
Finally, when you've digested all of that, you can delve into more details by checking out the ArsDigita Community System Documentation and looking thru the doc/sql directory, which defines the ACS data model.