and a fair degree of knowledge to code a forum like this one MeV1, and I certainly wouldn't class it as a "little project". Our forum was developed from scratch using various technologies, and it's fairly complex.
The forum pages are created dynamically by software called ColdFusion which runs on one of our servers. When your browser requests a page from our http server the call is passed to the ColdFusion server where the data is retrieved from our databases, put into a page that's constructed on the fly, and returned to the http server for display in your browser.
That's a simplified explanation, but you get the idea. It's pretty involved, ColdFusion contains several elements, one of which is a powerful tag-based web coding language called Cold Fusion Markup Language (CFML). There's also a 32-bit, multi-threaded application server that processes the Cold Fusion templates.
To be honest, unless you are an experienced web developer, and have a need for a complex forum populated by thousands of members accessing very large databases I wouldn't think it's something you want to get involved with. There's a reason why so many people use the 'off the shelf' solutions like Invision Forums - they're extremely good, richly featured, almost infinitely customisable, and inexpensive.