An operating system performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating networking and managing file systems.
Most operating systems come with an application that provides an interface to the OS managed resources.
Operating Systems have had command line interpreters as a basic user interface.
However, they recently have been implemented as a graphical user interface (GUI) for ease of operation.
Windows, Linux, and Mac OS are some of the most popular OS's.