Thousands of contributors began refining the Linux kernel and the operating system built on top of it. Linux went on to become, arguably, the biggest success story of the free-software movement, proving that the work of thousands of volunteers can create a piece of free software as powerful as one sold by any corporation.
In the early 1980s, the Unix operating system was already in widespread use throughout academia and businesses for both servers and workstations. It was being rapidly developed and deployed. Unix code could be made to run on hundreds of different types of computer hardware. This high level of portability was integral to its popularity.
But as it grew more complex, Unix (and its many Unix-like cousins) became increasingly saddled by licensing fees. Demand began to rise for a free operating system, something as powerful and flexible as Unix, that could be distributed and modified openly and freely without the encumbrance of commercial licenses.
To that end, Richard Stallman, a programmer at MIT, founded the GNU Project in 1984. Stallman and his collaborators began assembling the various pieces of a free operating system that would be compatible with Unix, strictly adhering to the idea that software should be not only be freely available, but also give its users the ability to freely experiment with its inner workings.
According to wired