The Linux kernel.
The operating system kernel is the core of every operating system. Linus Torvalds created the Linux kernel.
It means that Linux is a kernel for Linux-based operating systems. In the strictest sense, Linux itself is only the kernel, however I tend to view it as a full operating system core due to how most kernel spece works (Which tends to include not just the kernel, but most device drivers.).
There are many open-source operating systems. They include:AROSContikiDarwin (the core of Mac OS X)ECosFreeBSDFreeDOSGeckOSGNU HURDHaikuJNodeJXKolibriOSLUnix (not to be confused with Linux)MonaNetBSDOberonOpenBSDPlan 9ReactOSSyllableSystems based on the Linux kernel (Ubuntu, Debian, RHEL, Fedora)Systems based on the OpenSolaris kernel, including Nexenta and Belenix
An operating system is software that consists of smaller pieces of software that work together. Linux at its core is a kernel, which is the core of the GNU/Linux operating system distribution.
Kernel is the core component of any operating system whereas root is the super user in Linux OS.
Mac OS X uses a Mach kernel and FreeBSD utilities at it's core.
The kernel is the lowest layer of abstraction in any OS, including Linux. It takes care of the core components of a system, and manages the hardware.