Also known as an "OS," this is the software that communicates with computer hardware on the most basic level. Without an operating system, no software programs can run. The OS is what allocates memory, processes tasks, accesses disks and peripherials, and serves as the user interface.
Thanks to operating systems, like Windows, Mac OS, and Linux, developers can write code using a standard programming interface, or API. Without an operating system, programmers would have to write about ten times as much code to get the same results. Of course, some computer geniuses have to program the operating system itself.