The kernel is the privileged code that executes inside the operating system's protected kernel environment. An operating system as experienced by users contains much more than the kernel.

Kernel responsibilities

A kernel commonly provides low-level mechanisms such as scheduling, virtual-memory management, interrupt handling, device access and system-call entry points.

User-space services

Many services associated with an operating system can run as ordinary processes: login managers, graphical servers, network configuration services, logging daemons and desktop components.

applications and system services   ← user space
---------------- system-call boundary ----------------
kernel subsystems and drivers      ← kernel space
---------------- hardware ----------------

The boundary is architectural

Whether a service belongs inside the kernel is a design choice constrained by performance, isolation and complexity. Monolithic kernels place many subsystems and drivers in kernel space; microkernel designs deliberately move more services into user processes.

The operating system is therefore not synonymous with its kernel. The kernel is the privileged foundation on which a larger user-space system is built.