An operating system (OS) is the system software on a computer that manages the hardware and software resources available to it. Typical hardware resources managed by the OS include input devices like a keyboard and mouse, output devices like monitors and printers, network devices like routers and modems, and storage devices like internal and external drives. The OS also helps software work properly by allocating memory, processing execution orders, and much more.
There are many different features and components that make up a typical OS, such as the kernel, the user interface, networking services, and security services such as firewalls. The OS is usually pre-installed on the computer when you purchase it, but it can be replaced or upgraded after purchase. The three most common OSes for personal computers are Microsoft Windows, Mac OS X, and Linux. Each OS has a different look and feel, and users develop strong personal preferences, but most features are available in all of the OSes.
Windows is the most popular OS for personal computers, with more than 80% of the market share in October 2017. Mac OS X had 13% market share, and Linux had 2%. However, most servers run on a Linux OS, because they are relatively easy to customize.