You just bought a new hard drive for your computer to replace your primary hard drive. After backing your data up and, then, installing the hard drive you are now ready to install the operating system on it. What are the two main things does an operating system do?

The main purpose of an operating system is to allow the user to be able to access and use the computer hardware. So the operating system itself acts as an interface to all the computers components (hard drive, network card, memory, etc.). Having access to these components now allows you to run Software applications that utilize said components to do you work. So you could say 2 main things are:

1. An OS acts as an interface to the computer's hardware components and resources. (RAM, disk, Network card, etc.)

2. Serves as a platform to run software applications that utilize said components to perform a function. (Think internet explorer accesses network card which allows you to go out to the internet).

It manages the soft and hardware resources of the system. It also provides a stable, consistent way for applications to deal with the hardware without knowing all of the details of the hardware