In the 1980's, Unix was very proprietary and very expensive. Linus Torvalds, attending the University of Helsinki and frustrated by the licensing of Unix (MINIX, specifically), began a small project to build a Unix kernel which he could then freely use.
At the same time, the GNU project had been building utilities which they were releasing to the public as Open Source, a form of software socialism defying the high costs of software at the time.
Thus was born Gnu/Linux (as Linux is more properly known), a truly open operating system. All the source code was open to anyone who wanted it, several distributions were created, each with a separate idea of how to best build an operating system targeted to specific users.
Linux currently (2016) has the largest install base of any operating system due to its use as the basis for the Android phone OS.