the kernel
The kernel is the innermost, most trusted part of the operating system — the program that is always running and that has full power over the hardware. If the OS is the hotel staff, the kernel is the manager with the master key who can enter any room and operate any machine; everyone else has to go through them. Its name comes from being the core: strip away the menus, the windowing system, and the bundled apps, and the kernel is what remains as the true heart of the OS.
The kernel runs in a privileged mode (kernel mode) where it may execute special instructions ordinary programs cannot — directly touching memory, talking to devices, and switching which program runs on the CPU. It is loaded into memory at boot and stays resident the whole time the machine is on. Its responsibilities are exactly the core OS jobs: deciding which process gets the CPU next, handing out and protecting memory, mediating access to files and devices, handling interrupts from hardware, and answering system calls from user programs. Ordinary applications never run as the kernel; they request the kernel's services through the controlled doorway of system calls.
Why single out the kernel? Because the boundary between kernel and everything else is the most important security and stability line in the whole system. A bug in an ordinary app crashes that app; a bug in the kernel can crash or compromise the entire machine, since the kernel is trusted with full hardware power. That is why operating systems work hard to keep the kernel small, careful, and hard to fool — and why how the kernel is structured (monolithic, microkernel, hybrid) is a major design question with real consequences.
When your program calls read() to get data from a file, control transfers into the kernel: the kernel checks you are allowed to read that file, asks the disk for the bytes, copies them into your program's buffer, and returns. Your code never touched the disk itself — the kernel did it on your behalf, in privileged mode.
User code asks; the kernel, holding the master key, does the privileged work.
The kernel is not the whole OS — a full operating system also ships shells, libraries, drivers, and tools around the kernel. But the kernel is the part that runs privileged, and the user-vs-kernel boundary is the line everything else respects.