An operating system (OS) is a kind of computer program that helps a computer to interact with other machines or with people. An OS is not actually one single program but a group of small programs, like file managers, device drivers, and kernels. An OS can be small (like Damn Small Linux), or large (like Microsoft Windows). Some are meant for everyday use, like in a personal computer or a smartphone. Others are very specialized - for example, an ATM uses a kind of operating system.

An operating system has many jobs. It makes sure that all the programs share access to the computer's processor, memory, screen, input devices, and other hardware. Today, most operating systems also include a visible interface so the user can easily interact with it. An OS is also responsible for sending data to other computers or devices on a network.

Some examples of commonly used operating systems are macOS, Linux, and Android.

One of the first computers was ENIAC (Electronic Numerical Integrator and Computer), built by the United States during World War II.^[1] Making ENIAC do useful work was difficult. To program ENIAC, scientists had to move switches and cables around on the side of the computer. While this was an operating system of a kind, it is not really like modern operating systems.

The first operating system that looked and felt like today's operating systems was UNIX, made in 1969 by Bell Labs. With UNIX, people could program computers by typing on a keyboard. Many of its features were taken from Multics, an older operating system made in 1964.^[1]

Operating systems come in many different types. An OS might fit one or several of the types listed below, and the difference between the types isn't always clear.

A single-tasking system can only run one program at a time. A multitasking operating system can run more than one program at the same time. To multi-task, the computer lets each program take a turn using the processor.

Single-user operating systems don't let people create their own "accounts" on the computer - there is only one user. A multi-user operating system lets multiple users interact with the system at the same time.

A distributed operating system takes a group of distinct computers, which might be all over the place, and makes them work together like one single computer.

Embedded operating systems are very small OSes used in embedded systems. They are designed to operate on small machines, like the electronic part of a microwave, and they only do a few particular things.

Hobbyist OS are made by individuals or small groups. There are many online communities like OS dev wiki to learn how to make an OS.

• BIOS