Hey Davin,
So maybe I am missing something here. . .if I host domainname.com site on some host other than WordPress, does wordpress simply default to a CMS tool and I end up with mydomainname.com rather than mydomainname.wordpress.com? Hmm, given your comments re: Wordpress folders, I might be missing the point here though. . .
Slightly off topic but related. So I put mydomainname in its own wordpress folder and mydomain . . .never mind. I'm making it more confusing, right?
How do I install a PhpBB forum with a few clicks on a domain? Oh, so I just have another folder with another set of pages on the same domain but which contains my forum only! The Blog would reside on the same domain but in the blog folder. Then I put a link on the blog to the forum!
So how do I install a PhpBB forum with a few clicks on a domain again?

so one more thing, mydomain.com just is my home page at mydomain.com. so where does Wordpress fit in again?
Wow, my ignorance is really showing but I want to keep learning. . .
Thanks Davin,
Bella