Since the 1980's, before Linux, groups of Unix developers began to believe that software should be free and freely used. This idea gained considerable steam in the 1990's when Linux became the flag-ship project of the so-called Open Source movement. Open Source Software (OSS) is a 'new' software development paradigm that is in direct contrast to the traditional, proprietary development method. But these development styles differ in many ways, not just in whether the source code is available or not.
In the proprietary model, development occurs for years and the released product is generally very polished. When bugs are found or new features requested by users, it may be years before a new release incorporates changes. This is the 'release late-release rarely' development model. Intrinsic to this model is that software is a product, packaged for consumption and purchased by users as a 'finished product.' Key here is the role users play: consumers. One problem with this idea is that users cannot ever be completely removed from the development process, but their role in the proprietary development model is rather limited (in most cases).
Many OSS projects are done on a 'release early-release often' model. This means that the developers release software as it's being developed. The software is not viewed as a product, but a means to an end, such as an increase in productivity. Users get snapshots of applications and tools before they are complete (feature-wise) or polished, and changes are incorporated rather quickly. Users can input ideas, feature requests or report bugs. But even more importantly, users can modify the programs themselves to suit their needs if the development is not to their liking. This is key, and lies at the basis of the term 'Free' in Free Software as it is applied in the Open Source Movement.
Most OSS is 'free,' but the use of this term causes considerable confusion. Many believe that free means ``free as in lunch.'' This is only part of the story. While it is true that a large quantity of Open Source Software can be had for no money, the real meaning of free is ``free as in speech.'' That is, when a user uses free software, he is free to use it as he wishes. He can change it, give it away to his friends, or even, in most cases, sell it for profit. This is fundamentally different than most proprietary software for which one does not even own the product one purchased (but rather a license to use it under certain conditions).
A manager considering the appropriate platform to meet his business needs should consider this carefully. MS Windows is the archetypical 'software product,' complete with product lifecycles, end of support and a 'closed' format that means MS controls the development. Linux, being OSS, can be supported indefinately in-house if need be by in-house staff; one can obtain Linux for free, you never have to pay for an upgrade and Linux will never see ``end of life'' so long as one chooses to maintain it. Indeed, many OSS projects, especially big ones like the Linux kernel, have such large a user and developer base that even if the current maintainers dropped their projects, others would likely take over.
Advocates of proprietary software, such as Windows, like to posit that documentation is better than for OSS. This depends on the software being considered. I've seen really terrible documentation for both proprietary and OSS software, and I've seen excellent documentation for both as well. Linux itself is extremely well documented. To me, the documentation for Linux makes more sense, since with Windows, MS seems to redefine every term in computing to their own. Reading MS Windows documentation is like reading something written and proofread by insiders, marketing and legal corporate bureaucrats. To me, personally, the documentation for Linux and related software appears to be written by real people trying to solve the real problem they are addressing.
Finally, being Open Source, Linux is infinitely configurable. You can change anything about the Operating System you would like; you cannot do that with Windows; you have to take what is given to you.