Linux is a generic term that refers to a family of open-source operating systems that is based on the Linux kernel. The core of Linux operating systems, the Linux kernel, was originally developed by Linus Torvalds as an early shot to create a free, open-source version of Unix, which was closed and proprietary.
It has gained popularity because of the particular nature of how it was developed, and the effect that it had on the computer industry.
Supported early on its evolution by computer geeks, it started to grow popularity because it was free and allowed anybody with the required knowledge to use it without paying for any license. This new paradigm in software development of which the Linux is big part of is the reason Linux attracted the interest of the whole industry. It was disruptive that it had the potential to change the OS (Operating System) and software industry, which was then dominated by Microsoft Windows and Unix.
Where the traditional licensing model meant companies had to pay hefty license fees to use Operating Systems such as Windows or Unix, in the Linux scenario, companies get the OS for free, and would pay instead for support (if they need it). Linux provided a very low-cost alternative for many companies. Its development has also received enormous support from big industry players which led to massive improvements that made it a very viable and for some, also a better alternative OS. Now it is used in many industries and in many different devices.