Linux is an operating system (OS) closely related to Unix. Linux is an Open Source OS which means that anyone can access the original source code and modify it to suit their purposes. This has resulted in the many different flavors of Linux referred to as distributions (distros). Linux is noted for being supported by many free software packages that replace the proprietary software used in many operating systems. There are free and open source word processors for example.
Linux started as the GNU Project  which produced many software/utility packages for a future OS they wished to develop. Linus Torvalds created the Linux kernal, the heart of the OS, in 1991. Torvalds combined his kernal with the GNU software and Linux was born.
Linux as Utility
Many technicians supporting Microsoft Windows operating systems use a bootable Linux CD/DVD  to access hard drives to perform repairs and retrieve data that may be lost if their repair efforts require them to format and reinstall Windows. In addition, there are distros of Linux made to do computer forensic work with, these are used by experienced computer security people to examine and clean up computers infected with viruses or malware.