Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run.
Debian provides more than a pure OS: it comes with over 51000 packages, precompiled software bundled up in a nice format for easy installation on your machine.
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 or the FreeBSD kernel. Linux is a piece of software started by Linus Torvalds and supported by thousands of programmers worldwide. FreeBSD is an operating system including a kernel and other software.
However, work is in progress to provide Debian for other kernels, primarily for the Hurd. The Hurd is a collection of servers that run on top of a microkernel (such as Mach) to implement different features. The Hurd is free software produced by the GNU project.
A large part of the basic tools that fill out the operating system come from the GNU project; hence the names: GNU/Linux, GNU/kFreeBSD, and GNU/Hurd. These tools are also free.
Source from https://www.debian.org/intro/about