"The Debian Project is an association of individuals who have made common cause to create a free operating system. This operating system that we have created is called Debian GNU/Linux, or simply Debian for short.
An operating system is the set of basic programs and utilities that make your computer run. At the core of an operating system is the kernel. The kernel is the most fundamental program on the computer and does all the basic housekeeping and lets you start other programs.
Debian systems currently use the Linux kernel."
Debian is most well known for it's powerful and mature packaging system (known as dpkg and/or apt), and it's advanced policies.