An operating system (OS) is a software that manages computer hardware and software resources and provides common services for computer programs. It acts as an intermediary between the computer hardware and the user, allowing the user to interact with the computer and run applications.

The main functions of an operating system include:

1. Process management: It manages the execution of programs or processes, allocating system resources and scheduling tasks.

2. Memory management: It controls and organizes the computer’s memory, allocating and deallocating memory space for programs and data.

3. File system management: It manages the organization, storage, and retrieval of files on storage devices, such as hard drives or solid-state drives.

4. Device management: It controls and coordinates the use of hardware devices, such as printers, scanners, or network interfaces.

5. User interface: It provides a user-friendly interface for users to interact with the computer, such as a graphical user interface (GUI) or a command-line interface (CLI).

6. Security: It ensures the security and protection of the computer system and its resources, including user authentication, access control, and data encryption.

Examples of popular operating systems include Microsoft Windows, macOS, Linux, and Android. Each operating system has its own features, capabilities, and compatibility with different hardware and software applications.