What is an Operating System? Definition, Functions, and Types

An operating system (OS) is the main software that runs your computer. It controls the hardware, shares resources between programs, runs your apps, and gives you a screen, files, and folders to work with. Without an OS, a computer is just parts that cannot do anything useful on their own.

What Is an Operating System in Simple Terms?

An operating system is the layer of software that manages the hardware and lets your programs run on it:

• The middleman: it sits between the physical parts (CPU, memory, storage, devices) and the apps you open, so each app does not need its own hardware code.
• Always running: it loads when you turn the machine on and keeps running underneath everything you do.
• You already use one: Windows on a PC, macOS on a Mac, Android or iOS on a phone – each is an operating system.

Best for understanding: think of the OS as the manager of the computer – it decides who gets what and keeps everything working together.

What Does an Operating System Do? (6 Core Jobs)

An operating system handles six core jobs that turn raw hardware into a usable computer:

Best for beginners to remember: manage hardware, manage resources, run programs, handle files, keep things safe, and show a usable screen.

What Is the Kernel?

The kernel is the core of the operating system – the part that talks directly to the hardware:

• The engine room: it runs at the most privileged level and controls the CPU, memory, and devices directly.
• Its main work: scheduling which program uses the CPU, handing out memory, and routing requests to device drivers.
• Everything sits on top: the desktop, file tools, and your apps all run above the kernel and ask it for hardware access.

How Is an Operating System Built? (The Layers)

A modern OS is organised into layers, from the bare hardware up to the apps you click:

1. Hardware layer: Physical CPU, RAM, storage, and peripherals. The OS does not belong to this layer but manages it.
2. Kernel layer: The core OS component running in privileged mode. Handles process scheduling, memory management, device drivers, and system calls.
3. System libraries and APIs: Standard libraries (glibc on Linux, ntdll.dll on Windows) that translate application requests into kernel system calls.
4. User space: Applications and user-facing processes running in unprivileged mode, isolated from direct hardware access.

Best for the big picture: requests flow downward (your app to the libraries to the kernel to the hardware) and results flow back up.

What Are the Main Kernel Types?

Operating systems use one of three kernel designs, differing in how much code runs in the privileged core:

Best for: most everyday systems you use (Windows, macOS, Android) lean on monolithic or hybrid designs for performance.

What Are Examples of Operating Systems?

Five operating systems cover almost every device people use today – across desktops, phones, and servers:

• Windows 11: the most common desktop/laptop OS, made by Microsoft (Windows 10 lost support in late 2025).
• macOS: Apple’s desktop OS, running on Mac computers.
• Linux: a free, open-source OS family (Ubuntu and others) that powers most servers and the cloud.
• Android: Google’s mobile OS and the most-used operating system in the world; it is built on the Linux kernel.
• iOS: Apple’s mobile OS for iPhone.

The table below compares the five by current version, kernel type, where each is used, and rough market share:

Which Operating System Is Used Most?

Counting every device, Android is the most-used operating system in the world, with Windows leading on desktops:

• All platforms combined: Android is #1 at about 35%, ahead of Windows at about 30% (StatCounter, 2026) – because phones outnumber PCs.
• Desktops: Windows dominates at roughly 66%, macOS around 7-15%, with Linux rising past Windows 10’s end of support.
• Phones: Android holds about 70% and iOS about 29%; together they run over 99% of all phones.

Best for context: there is no single winner – it depends on whether you count desktops, phones, or every device at once.

What Are the Types of Operating Systems?

Beyond the products you know, operating systems are grouped by how they are used, into five types:

• Batch OS: Processes jobs sequentially without user interaction during execution. IBM z/OS on mainframes processes large transaction batch workloads overnight.
• Time-sharing OS: Shares CPU among multiple interactive users using time slicing. UNIX was the first widely deployed time-sharing OS, supporting hundreds of simultaneous terminal users on a single PDP-10.
• Real-time OS (RTOS): Guarantees response to events within defined time bounds. FreeRTOS and VxWorks are RTOSes used in embedded systems and industrial control where missed deadlines cause physical harm.
• Distributed OS: Manages multiple networked computers as a single unified system. Google’s Borg and Kubernetes are production distributed OS platforms managing millions of containerized workloads across data center clusters.
• Network OS: Provides network services as its primary function. Cisco IOS and Juniper Junos are network operating systems running on routers and switches, handling packet forwarding at terabits per second.

Best for: the OS on your PC or phone is a general-purpose, time-sharing system; the others run servers, machines, and networks behind the scenes.

How Does an OS Run Many Programs at Once?

It uses a scheduler that rapidly switches the CPU between programs and virtual memory to stretch RAM:

• Time-slicing: the scheduler gives each program a tiny turn on the CPU (often 10-100 ms), so many appear to run at the same time.
• Fair turns: common methods include round robin (equal turns) and priority scheduling (urgent tasks first).
• Virtual memory: if RAM runs low, the OS parks less-used data on the drive and loads it back when needed – so programs can use more memory than is physically installed.

Responsiveness varies between platforms, and a comparison of the fastest operating system options explains why a lean kernel and fewer background services produce quicker boot and load times.

Last Thoughts on Operating Systems

The operating system is the foundation every other piece of software stands on. It manages the hardware, shares resources, runs your programs, organises your files, keeps things secure, and gives you a screen to work with – all at once, every second the machine is on.

You do not need to know kernels or scheduling to use a computer, but knowing that one piece of software is quietly running the whole show makes the rest of computing far easier to understand. Whether it is Windows, macOS, Linux, Android, or iOS, the job is the same.

Key Takeaways:

• An operating system manages hardware resources and provides services to application programs through 6 core functions: process management, memory management, file system management, device management, security, and UI.
• Three kernel architectures exist: monolithic (Linux), microkernel (seL4, QNX), and hybrid (Windows NT, macOS XNU).
• Windows holds ~66% desktop market share; Linux dominates servers at ~96% of cloud infrastructure; Android holds ~70% mobile share and is the most-used OS overall across all devices.
• Virtual memory uses paging to give each process a 16-exabyte address space (64-bit), mapped to physical RAM via page tables and the TLB.
• The Linux kernel 6.x contains approximately 27 million lines of code; seL4’s verified microkernel contains approximately 9,000 lines.
• Round Robin scheduling with quantum sizes of 10–100ms is the basis for modern interactive OS schedulers including Linux CFS.

Frequently Asked Questions (FAQs)