You mean you're running one OS on top of another's kernel?

No, I'm not that crazy .
Debian per definition is not a complete OS. Debian rather means and provides the management tools and programs/software necessary to administer and use a system i.e. it's not a full OS, but one part of it. To make up a full OS, a kernel is necessary, and Debian primarily uses the Linux kernel, hence why the most common flavour of Debian is indeed Debian GNU/Linux.
But there are also some non-Linux ports of Debian, i.e. ports of the tools and utilities that it consists of and uses to manage the system, to other kernels, FreeBSD's being one of them. Other non-Linux Debian ports use the Hurd, NetBSD or OpenSolaris kernel.
From Debian GNU/kFreeBSD's page:

(GNU userland = important Unix commands and libraries to perform basic operations.)

An analogy would be if I decide to build my own car, I can build the chassis , the steering wheel, the brakes, etc. i.e. almost everything but in the end, I'll still need an engine because without one, the car will still be pretty much worthless and the other way round, it's the same – what should I do with an engine, but without a car to put it into? So, I may decide to use an engine built by, say, BMW or another one built by Mercedes. The car will still look and drive roughly the same, but it'll have another engine in it and I may have to assimilate some of the parts to be compatible with the other engine.

I hope it's clearer now

Cool, thanks for the explanation I still think you do some crazy stuff though!

No problem. Well, "to have fun" is a widely adaptable expression, isn't it?

AbiWord, displaying a very intelligent sentence, on the Xfce desktop, running on Debian GNU/kFreeBSD:

Guess I'll post mine...