What is Debian?

Linux is just the kernel of your operating system.
You need a lot of applications around.
Those collections of software around the Linux kernel is called operating system or distribution.
Companies who build such distributions are called distributors.
They make money by selling their distribution in boxes, doing support and training.
You might know Mandrake, RedHat, SuSE
You might know Mandrake, RedHat, Novell etc.
Debian is just one of them.