OpenACS is first a toolkit for building applications. The applications generally have an HTTP interface provided by the AOLserver. That is their similarity to websites. However, unless you were going to punt all you know about web development, I don't know that I'd advse it for building mere websites. I've been using it for a year now, and I still don't understand some things that are pretty basic to some members of the community.
If it were just a neat way of using TCL from a webserver you were looking for, you've come too far. Go back 100 yards and pull into the AOLserver parking lot. It's a darned fine webserver. With the exception that it doesn't have dumb-simple virtual hosting, it's probably the best webserver I have seen. The fact that PHP and Perl aren't trivially supported is a plus in my book :)
Now, to understand OpenACS, I believe you need to grok the notion that little you do in the filesystem matters. What matters is that you get everything running, and by interacting with the administration interface, you construct a hierarchy of subsites, mount applications onto these subsites, and change their parameters. At no time do you directly create files in the filesystem. This last has beem my greatest difficulty in coming up to speed with OpenACS. It may be that for a particular situation, one needs to create a file on the filesystem. However, I've never gotten a clear answer on when and where that is.
Anyway, after I started this reply on Friday, I realized that I might be able to contribute to this little dance party here. I created some movies of the configuration of OpenACS from the point where the AOLserver and OpenACS kit are installed and ready to go. Now, getting to that point isn't trivial. In general, "You can't get there from here." the Debian version of GNU/Linux has a ready to run kit, I believe, as does FreeBSD. I use Solaris, so all bets are off. The intersection of Solaris and OpenACS users may well have been the empty set before I came.
So, if you'd like to see how the initial set-up and config of a site, sub-site, and user go, go to http://www.webrelay.net/download/oacs/ and grab the three avi movies there. I think they'll be helpful. A warning: The videos are not compressed. I'd welcome a private note as to how to do that. They are also not ready-for-prime-time. More like dress rehersal. I hope to revise them soonish. Oh, and if you'd like to get this kit up on a Solaris/SPARC system, the svr4 packages in that dir will get you going. You'll probably want to get them with wget or fetch, as they are dirs, not files.
Anyhoo, I realize that this doesn't answer all of your questions, but I hope this helps.
Cheers!