Linux

The Linux kernel is a Unix-like operating system that was built under the open source model for development and distribution. The name GNU/Linux term is preferred to the Free Software Foundation, and is commonly used, due to the fact of the compiler that builds the kernel and other tools was written by Richard Stallman, founder of the GNU project and the Free Software Foundation.

History
Linux was originally developed for the Intel x86 platform. It has now been ported over to many different platforms, including ARM, and others. It has even been ported to itself, by running it in usermode, instead of in kernel mode.