The UNIX operating system was at one time the most widely-used Operating System on computer mainframes. At the time, personal computers were rare, so users would use a computer terminal which would connect to the mainframe (think of using a modern server today, like a Chromebook does), and use its resources. UNIX was generally regarded as the best available Operating System, but came with expensive licensing fees, which prompted many people to create similar Operating Systems using open-source tools and following similar standards to those practiced by the UNIX team. 

While there are still some UNIX systems in use today, the majority of its present influence comes from its prevailing influence over more modern Operating Systems. For example, DarwinOS-descended languages, such as MacOS, iOS, iPadOS, WatchOS, and tvOS are all based on UNIX to some degree. And the most widely used Operating System today, Linux, was directly based on and designed to replace UNIX. Linux is the underlying Operating System that powers Android Phones, Chromebooks, and nearly all smart appliances today. 

Microsoft DOS is one of those stories where the story is almost as interesting as the product itself. In the early days of Microsoft, Bill Gates had to bring an Operating System to market if he wanted to be able to sell software on the new IBM Personal Computer. In response, he acquired 86-DOS from Seattle Computer Products, which was a nearly-finished Operating System. Gates was able to complete the Operating System, which he renamed MS-DOS, and was able to present it to the IBM team, which ultimately led to its inclusion on the IBM PC. 

While MS-DOS is pretty rare today, mostly found only in the oldest of medical equipment, it led to the creation of Windows, which is today the largest desktop Operating System in the world, and is used as the primary Operating System by most governments and corporations around the world for users' machines. 

NeXTSTEP was an Operating System developed under Steve Jobs at the company NeXT, following his firing from Apple. The NeXT Operating System was unique in that it was the first Operating System of its time to be both practical and to possess a true Graphical User Interface, or GUI. The inclusion of the GUI made the use of NeXT computers more accessible to more people, and played a major role in the development of the Internet, because it allowed for the development of graphical applications for the NeXT machiens used in Cern. 

Upon his return to Apple, NeXTSTEP would be converted to the now open-source DarwinOS, which in turn would eventually become MacOS. MacOS would later become Mac OS X, which became the foundation for iOS, tvOS, WatchOS, and later iPadOS (iPads originally ran on iOS). While likely the lest well-known of the Operating Systems included here, NeXTSTEP had arguably the greatest impact, because it led to the creation of the Internet based around Graphical Interfaces, and likely accelerated the process considerably.