Linux is a Unix operating system clone, but one that was written completely from scratch by Linus Torvalds and a small team of hackers from all reaches of the world. Linux aims to be POSIX compliant, as well as UNIX Spec compliant. Linux is the kernel, while other GNU software makes up the bulk of what we call "Desktop Linux."
Linux is at the heart of every Android device
Linux is a full-featured Unix, with true multitasking — including network multistack tasking with both IPv4 and IPv6. Linux also offers proper memory management (including virtual memory), shared libraries in the system folders, on-demand loading and copy-on-write executables. About 70-percent of Internet servers, and 95-percent of "supercomputers" run the Linux kernel. Since Android is Linux, about 80-percent of mobile devices also run Linux. You'll also find Linux in your coffeemaker, or your DVD player. Linux is extremely scalable, easy to build and program for, and very popular.
Android devices use the Linux kernel, regardless of who built them. There are also several GNU tools we would see on desktop Linux in our phones and tablets to control things like wireless radios and networking. Google has used much of their own code as the middleware and user interface in Android, but at it's heart Android is still Linux. The Linux kernel is free open-source software, and is covered under the GPL.