The Kernel
A kernel is a computer program that is the core of a computer's operating system, with complete control over everything in the system.
The code of a kernel is loaded into a protected area of memory, preventing it from being overwritten. The kernel runs processes and handles interrupts in kernel space. In contrast, everything a user does is in user space: from writing text in a text editor, to running programs. This separation prevents user data and kernel data from interfering with each other and causing instability and slowness.