Linux is not the whole system. It is just the core.

The core of an operating system is called the “Kernel”. It runs in a special, privileged mode, in which the CPU protects the kernel from regular user-space programs.

While there are different approaches on what kind of functionality should (or should not) be included inside this core, at least the following areas are handled by the kernel: Scheduling (Multi-tasking), interprocess-communication, and memory-management.

Linux is a monolithic kernel that also includes device-drivers and the like. Wikipedia has detailed information on kernels.

Created by stw