I didn't write this forum from scratch. Although, I've done a good deal of programming to change it as I saw fit and i've written a handful of forums in my lifetime. This forum had a much richer feature set than ones that I have written in the past. The source code for the forum (as well as the portal) is part of an open source project I've been loosely involved on and off with over the last year or so..
To explain how it's written, well - that's a little more than I can get into in this forum but.. Essentially, the webpages for the forums are programs that read and write data into a database. The difference is, instead of a user interface like MS Access or Word, the Interface is ASP.Net which interprets “stuff” as HTML so your web browser can view it.
So.. if you're asking how to get this forum added to your webpages.. There is definiitely a good deal of programming required to add it. And to modify it you really need a good solid understanding of visual studio.net and the ASP.Net platform... But, you're right - there are plenty of perfectly good forum packages out there.