Anyone with a minimal knowledge of computers knows about Microsoft Windows®. A much smaller audience knows about Linux, or have even heard about Linux. Yet, everyone who uses a Droid phone, or an Android tablet, is working with the mobile device version of Linux. While teaching Linux kernel debugging courses, students often reminded me that embedded Linux represents the largest install base. Embedded Linux appears in all sorts of smart devices, such as routers, televisions, and industrial control devices. Thus, Linux appears in everything from smart devices to main frame computers. Linux is not limited to Intel based computers, but works with a wide variety of CPU and GPU chips. Linux works on old computers and new computers. There is no need to buy a new computer just to support the latest version of Linux. So, what is Linux?