An operating system (OS) is a program that manages a computer's hardware and software, and allows users to interact with the computer. It's the most important software on a computer.

• Manages hardware and software: The OS controls the computer's memory, processes, and peripheral devices.
• Coordinates programs: The OS ensures that all programs have access to the computer's resources, such as the CPU, memory, and storage.
• Provides a user interface: The OS allows users to interact with the computer through a user interface (UI), such as a command-line interface (CLI) or a graphical user interface (GUI).

• Mobile operating systems
  These are used on mobile devices like smartphones and tablets. Examples include iOS and Android.
• Desktop operating systems
  These are used on personal computers. Examples include Windows, macOS, and Linux.
• Real-time operating systems
  These are designed to respond to events in real time. They're used in applications like robotics, industrial control systems, and embedded systems.

While most computers come with an operating system pre-installed, it's possible to upgrade or change it.