Although I haven't worked on a corporate website in a year, and have never really worked on the sales end of marketeering, it was the case in the early 2000's that everyone was obsessed with three tier architecture.

Three tier architecture is a joke. Any software project can be "transformed" into a 3-tier architecture. Why is 3-tiers so magical? Cause people like the number 3, I know that I do. With many companies sending woefully unequiped people out to buy software services they latched onto whatever stupid concept was being pimped at the time. For the same reason lots of people are addicted to Java, not that Java is a bad programming language, it isn't, but it certainly isn't the best solution to every problem, and there is probably more bad Java programming out there then there is bad LISP or Python programming.

If you ignore my tirade against the marketing hype of 3-tiers, you can see quite clearly that OpenACS adopts a three-tiered architecture (of course so does a ham sandwich if you view it from a certain angle -- there is the bread tier, the meat tier, and the mustard tier).

• Web Client: The web client is the user's browser. While most (evil) web designers like to design their products to work with Internet Explorere's version of HTML, one should always assume nothing about the user's web client. Maybe they are kicking it old school and using a text browser like W3, an old relic like Netscape 3, or maybe they are an OSS enthusiast like myself and using Mozilla. The general assumption in OpenACS is that the user may be using almost anything that adheres to the HTTP and HTML standards, which IE does not.
• Web Server (AOLserver): Although HTTP is a relatively simple protocol people want to be insulated from it as much as possible, just as they don't want to manually binding the the socket etc. It's responsibilities (at the highest level) are to talk to the Web Clients and provide the potential of communicating with the database.
• RDBMS: The Relational Database Management System is used for storage of data. Don't let the picture fool you, OpenACS is designed primarly to work with PostGreSQL, not ORACLE.

Image stolen from Phil's book