An operating system (OS) is the fundamental software that manages the hardware of a computer or mobile device and provides the environment in which applications run. It is the intermediary between the physical components (processor, memory, disk) and the user. Without an operating system a computer would be nothing but a non-functioning collection of parts, because there would be nothing to tell them how to work together.
The operating system is the heart of every digital device. Its main jobs are allocating system resources to individual programs, managing files, ensuring data protection and providing a user interface (graphical — GUI, or textual — CLI).
These operating systems currently dominate the market:
For web analytics and digital marketing, knowing a visitor's operating system is key (in Google Analytics, for instance), because it makes it possible to optimise a site for particular platforms or target advertising at owners of more expensive devices (users of the newest iOS, say).
Imagine you commission an agency to build a new mobile application for your e-shop. For the app to work correctly, the developers have to take the differences between operating systems into account. While on Android the “back” navigation button is part of the system, on iOS that button has to be built into the app's design. If a developer ignored the specifics of the operating system, users on one of the platforms would be confused and would stop using the app, which would lead to a drop in your revenue.
