A proprietary operating system is one which a particular company conceptualizes, designs, develops and sells. Examples of proprietary operating systems are Windows and Mac OS X. These operating systems are designed and sold by these companies and are not meant to be tampered with or tweaked by users. Open source operating systems allow the user to tweak and change it after it is downloaded. Examples of open source operating systems are Linux for personal computers and Android for mobile devices.
Simplified User Experience
For non-tech-savvy individuals, proprietary operating systems like Windows and OS X make the overall user experience simpler and smoother. The wider range of options available in open source reduces the out-of-the-box appeal of the product as it requires users to set preferences and configure the system to their liking.
User Multiplier Effect
The multiplier effect is the process wherein more and more users are drawn to a specific product -- in this case, an operating system -- based on the increasing number of people already using it. More users means developers are more inclined to design software based around the operating system. This in turn spurs more users to switch to the operating system. Proprietary systems can have an advantage in courting users because of the advertising capabilities of the large companies designing them.
A proprietary system may be frustrating for individuals who like to control and tweak various aspects of their operating system. Companies who design proprietary operating systems may make it intentionally difficult, and sometimes impossible, to change or even view the source code. Software companies generally want to protect the integrity of their product and prevent individuals from stealing proprietary knowledge by viewing sensitive source code.
Operating systems are often designed to work with a fixed set of hardware specifications. Open source operating systems are designed to work on just about any computer. This has historically not been the case for proprietary systems. For example, it is only since the introduction of Intel processors into Mac hardware that the computers have been able to run Windows. Furthermore, while Android has been designed to work on a wide range of hardware specs, iOS can only run on the iPhone.