What is an operating system?
The desktop that greets you when a computer starts, and the screen that launches apps on a phone, are both the work of the same invisible labourer: the operating system. So what exactly does it do?
Short answer
An operating system is the fundamental software bridging a device's hardware and its applications. It shares out memory and processor time, organises files, presents the interface you see on screen and manages security. Windows, macOS and Linux are the most common examples on computers; Android and iOS on phones.
The shortest definition: manager of resources
Dozens of programs run at once on a device, but the processor, memory and drive are finite. The operating system is the manager that shares these limited resources among programs: it decides how much memory each application gets and which task takes its turn on the processor.
Thanks to that management you can write a message while listening to music and downloading a file in the background. If one application crashes, the operating system can close it and keep the others running.
From files to interface: the core jobs
The visible face of an operating system is the interface: the desktop, icons, windows and the home screen on a touchscreen. Out of sight, the file system is at work: the operating system keeps track of where your documents sit on the drive and which folder they appear in.
It also handles communication with the hardware: the document you send to the printer, the image coming from the camera and the sound going to your headphones all pass through the operating system by way of small pieces of software called drivers. Security layers such as user accounts, permissions and the lock screen are its responsibility too.
You can see traces of these jobs on your own device: the task manager or system monitor on a computer lists the programs running at that moment and how much memory each is using, while the settings app on a phone shows which applications the battery and storage are going to. These screens are the windows opened to you onto the operating system's invisible bookkeeping.
The common operating systems
On desktop and laptop computers the three most widespread systems are Windows, macOS and Linux. Windows runs on devices from many manufacturers; macOS is found only on Apple computers; Linux is open source and serves both on personal computers and on the great majority of the servers behind the internet.
On phones and tablets there are two large systems: Android and iOS. The reason the same app has a separate version for each is that applications are developed for a particular operating system. Choosing a device is therefore also choosing an operating system and an ecosystem.
That variety affects your files as well: a document or photo will open on most systems without difficulty, while some programs are written only for particular ones. When changing devices — moving from a Windows computer to a Mac, say — what actually travels is your files; researching the equivalents of the programs you are used to before the move makes the transition smoother.
Updates and versions
Operating systems are living software: bugs are fixed, security holes closed, new features added. Version numbers track that evolution. Postponing updates does not break the device at once, but leaving known holes unclosed accumulates risk over time; the details are in the updates guide.
Every device's update support ends one day. Going online with a system that is no longer supported is like living with a door whose lock cannot be changed; not using such a device for sensitive matters such as banking is a sensible starting point.
What this knowledge is good for day to day
Knowing the operating system puts many terms in their place: an app being “Android only”, a game “requiring Windows”, or the permission screens in your phone's settings all make more sense. Knowing that the operating system is also the layer governing app permissions makes the steps in the app permissions guide easier to follow.
It helps with troubleshooting too: the reason restarting a device solves so many problems is that the operating system clears memory and makes a clean start.
One more term worth placing: a factory reset erases all the data on a device and returns the operating system to the clean state it was in on day one. It is the last resort for stubborn software problems and a required step before selling or handing on a device. A backup beforehand is essential; the process cannot be undone and removes personal data from the device.
Frequently asked questions
Does a computer work without an operating system?
It powers up but offers nothing usable; running applications, opening files and seeing an interface on screen all require an operating system. Even a brand-new drive has an operating system installed on it first.
Can Windows and Android be on the same device?
Generally no; each device comes with the system suited to its hardware. Some enthusiasts install more than one system on a computer, but for everyday use staying with the device's own system is the least troublesome route.
Does an operating system update delete my data?
Under normal circumstances it does not; updates are designed to preserve existing files. Even so, taking a backup before a major version change is a common and well-placed precaution against the unexpected.
Is Linux only for experts?
No; there are versions today that are easy to install and use. Still, in terms of broad application and game support, Windows and macOS offer a more familiar experience for most home users.
This guide draws on the official documentation of operating system makers and on the common definitions found in introductory computer science sources.