An operating system, or "OS," is software that communicates with the hardware and allows other programs to run. It is comprised of system software, or the fundamental files your computer needs to boot up and function. Every desktop computer, tablet, and smartphone includes an operating system that provides basic functionality for the device.
Common desktop operating systems include Windows, OS X, and Linux. While each OS is different, most provide a graphical user interface, or GUI, that includes a desktop and the ability to manage files and folders. They also allow you to install and run programs written for the operating system. Windows and Linux can be installed on standard PC hardware, while OS X is designed to run on Apple systems. Therefore, the hardware you choose affects what operating system(s) you can run.
Mobile devices, such as tablets and smartphones also include operating systems that provide a GUI and can run applications. Common mobile OSes include Android, iOS, and Windows Phone. These OSes are developed specifically for portable devices and therefore are designed around touchscreen input. While early mobile operating systems lacked many features found in desktop OSes, they now include advanced capabilities, such as the ability to run third-party apps and run multiple apps at once.
Since the operating system serves as a computer's fundamental user interface, it significantly affects how you interact with the device. Therefore, many users prefer to use a specific operating system. For example, one user may prefer to use a computer with OS X instead of a Windows-based PC. Another user may prefer an Android-based smartphone instead of an iPhone, which runs the iOS.
When software developers create applications, they must write and compile them for a specific operating system. This is because each OS communicates with the hardware differently and has a specific application program interface, or API, that the programmer must use. While many popular programs are crossplatform, meaning they have been developed for multiple OSes, some are only available for a single operating system. Therefore, when choosing a computer, make sure the operating system supports the programs you want to run.