Linux is not an operating system unto itself, but rather a kernel—a core component that manages hardware resources. Android uses the Linux kernel, but replaces the traditional GNU userland with its own runtime, libraries, and system framework.

Many users run Linux-based systems every day without realizing it. Through a peculiar turn of events, the Linux kernel combined with Android’s userspace is often simply called “Android,” and many of its users are not aware that it is built on Linux at its core.

There really is Linux in Android, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine’s resources to the other programs you run. The kernel is an essential part of the system, but useless by itself; it can only function in the context of a complete operating system.

Android is normally used in combination with the Linux kernel: the whole system is basically Android/Linux, a Linux-based operating system with a distinct userspace, not a GNU/Linux system like traditional desktop distributions.