A brief introduction to the early history of computer software
To better understand how the software world works it's important to get a clear idea of what are the origins and the strategies of each operating system and how history unfolded to make things as they are today.
At the beginning computers were developed as science projects by universities and governments. They were expensive, very big and only usable by experts. This was the innocent era of computer history, when developers were a small community that worked together to make computers do specific tasks with no interaction of companies or the public (= large amounts of money). The system they used was mainly UNIX, developed by AT&home appliance at Bell labs in 1969. The hardware was a collection of components assembled in very large rooms, each lab with their own specific and crafted mix of hardware.
The first company that started making computers was funded by two students in the mid 70's, Steve Wozniak, a genius geek, and Steve Jobs, a genius salesman. Together they created a market for an unknown product at that time: the Apple computer. They created the hardware configuration of the personal computers and also the software, inspired by UNIX, to make the most reliable, powerful and user friendly computer that they could. The strategy was to make a product that looks pretty much like any other home applliance, simple and useful.
Soon after, hardware companies started manufacturing these personal computers by their own and were desperately in need of a user friendly operating system. That's where Bill Gates and Paul Allen come into play. They decided to provide the software that would go into all the hardware in need of an operating system. To be able to do business with hardware vendors they had to produce quickly an operating system that could work on these machines. Due to time, technical and financial limitations they bought In 1980 the "QDOS" (Quick and Dirty Operating System) and transformed it into DOS (Disc Operating System) to deliver the software that IBM needed for their hardware. Microsoft suffered, from the very beginning, of a technical weakness but got their way around by finding the good business model: installing their software into every piece of hardware they could put their hands on. The strategy was to make their software the only one around by implementing it on computers from factory.
In the early 80's, Richard Stallman, a computer programmer working at MIT realized that all the software produced at that time was proprietary. Being a computer programmer working and tweaking with UNIX all day, he did not accept the fact that he simply had no access to the software source code that was being produced. Unsatisfied with the new way of building and selling software he announced in 1983 the GNU project. The idea was to create a free operating system based on UNIX with whoever would want to join and participate. The only essential piece of software they were not able to produce efficiently was the kernel. The kernel is the basis of the operating system that is used to allocate hardware resources to every software running on the computer. The reasons for that is that the GNU kernel (called Hurd) was a far too ambitious project. It eventually got operational in 2001 but the world could not wait that long. Ten years before that, in 1991, Linus Torvalds, like many others, a student, managed to write a kernel called Linux, and released it to the public. People immediately started adapting all the free software from GNU to the newly released Linux kernel forming thus an entirely Free Software operating system.
Today (2010) about 90% of computers run with windows installed on it. All hardware vendors (with some very few exceptions like Apple Inc.) sell their computers with Windows pre-installed. These companies hardware is legally tied with Microsoft software and sometimes they just cannot sell another software or even make it impossible (or very hard) to uninstall Windows. Mac Computers are about 7-8% of the market share and their computers are also only sold with Mac OS. Apple goes even further by making their software only installable in Apple hardware. Linux is running in about 1% of consumers desktops (this is already several millions of computers) though it had an amazing success with Internet servers (it represents about 70% percent of servers operating system) and super computers.
We can expect that GNU/Linux OS and, more generally, FOSS, is going to gain marketshare rapidly in the future for it's amazing diversity and adaptability.