A group of common software bundled together and installed onto hardware to run applications. The OS comes with a kernel to communicate with physical hardware. Windows, Mac OS X, Linux are the common operating systems (OS) for personal computers. Android, iOS, Windows Phone are the most common operating systems for smart phones.
The operating system is essentially a software platform. It also acts as the bridge between the physical hardware of a computer and the virtual software. For example, when you a run a video game the OS allocates resources for it. The operating system tracks how much processors and memory are used. Your video game will load models and textures onto the graphics card. This allows the game/software quick access to those things.