What does an operating system actually do?

An operating system is the software that manages a computer's hardware and gives every other program the common services it needs. Think of it as the building manager of a big apartment block: the tenants (your apps) cook, work and play, while the manager makes sure the power, water and doors work and that nobody moves into a neighbour's flat.

It does three jobs. First, it shares the machine out. The operating system gives each application a share of the processor's time and of the memory, so that one program cannot hog everything. Second, it keeps programs apart, so that a fault or a security hole in one of them does not spill into the others, while still letting them talk when they should. Third, it hides the messy details of the hardware, so a programmer asks for "a file" or "the network" without knowing which disk or which network chip the computer has.

That last job is why the same app can run on many different computers without being rewritten.

The early days show why all this was invented. The first computers were programmed with plugboards or punch cards and had no operating system whatsoever. Later mainframes still needed human operators who did by hand what a modern operating system does, such as deciding which program runs next. Today the manager is software, and it sits inside nearly everything with a computer in it, from phones and game consoles to web servers and supercomputers.