I think the way you're conceptualizing the forum isn't quite right. You need three things to make a web forum: (1) a web server; (2) a database server; and (3) a collection of PHP/ASP/whatever pages that uses the database and are served to users through the web server.
Each is a separate component and installed on its own.
Most often, people use Apache for the web server. There's an RPM for that (if it's not already installed on your CentOS box).
Also, many sites use MySQL as the database server. There's an RPM for that as well (and again, it may already be installed on your box)
Lastly, the collection of pages will be what runs the forum. Since it's just a collection of files, you don't "install" them per se. Generally, you download a tarball, unpack the tarball somewhere for your web server to see (usually /var/www/<something>), and then open a web browser to your server and the location of the files (for example: "http://myserver/<something>"). That will kick off a sequence of web-based "install" pages for you to do basic configuration of the forum.
The most difficult part of the process would probably be creating a database user and a database for the forum to use (but only because SQL syntax is a little hard to understand the first time you see it).
Anyway, that's usually it aside from your own customizations.
As for LinuxQuestions, I believe it runs a heavily-modified version of vbulletin
. Note that vbulletin is paid-for software. I used FUDforum
before and had no complaints. The FUDforum developers run their own forum
(of course) so you can see it in operation. FUDforum is also free.
But there are lots of others. You might also be interested in looking at CMS systems. I just recently looked at some free ones: Joomla!
, and Typo3