In 1991 in Helsinki, Finland, Linus Torvalds began work on a project that was, in his words, "Just for fun". This project eventually became The Linux Kernel. It was never intended to be anything special, just a tool for him to access the UNIX servers at a nearby university. He wrote it specifically for the hardware that he was currently working with, and it wasn't dependant on an operating system. After awhile, he realized that what he had inadvertently created was itself an operating system kernel.
Torvalds modeled his creation after a type of UNIX called Minix. The code for Minix was open source, but modification and redistribution was not allowed, therefore Torvalds' kernel contained no copyright code. Although it was modeled after UNIX, it wasn't UNIX. After he discovered what he had created, he posted the following on Usenet:
"Hello everybody out there using minix - I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since april, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things)."
Clearly at this time Torvalds didn't realize just how important his kernel would be to the open source software movement that had really started to gain influence around that same time. The Free Software Foundation, most known for their GNU Project which began development in 1983, was seeking a kernel to make their dream of "a sufficient body of free software ... to get along without any software that is not free" a reality. Yes, the goal was a complete operating system plus extra software tools that were all open source and protected by the GPL. In 1992 they discovered Linux, and GNU/Linux quickly progressed down the path that led it to where it is today - a massive body of free software that does far more than "get along without software that is not free".