Operating system

Early days when computer emerge there was programmers who write software to it. most of them were in low level languages like machine language and they were machine specific i.e. each machines software were written to it separately. Every machine use different sets of hardware and they have to handle differently so single program could not work on every machine programmers have to be written each machine separately.

These programs consist how to handle details like hardware how to send your report to printer i.e. instruction on printer how to print and sound instruction to play sound and instruction on display so they can be displayed correctly multi-tasking is a dream  they never even thought of and computers are assign single task. With the change of the hardware the software have to be change to adapt the needs of it.
Then people began to develop common platform to a compute that will manage hardware (takes care of how to handle and send instructions) and also resource management which make image of operating systems. The other software could be run on these systems depending on them so they need not to worry about the hardware or resource management tie OS did it for them. But still the OS depend on hardware and each machine have to be coded OS separately

With this things like DOS (Disk Operating system emerge) and a smart guy call bill gate had the chance to get hold of one of this he buy the system with the cording from the developer and did something miraculous, he separate the operation of hardware form the operating system so you have need not to code entire OS but the parts specifically handle that hardware which you may know now as drivers with this OS could run on multiple computers with minimum change OS are built to multi task, that is more than one process could run on single machine this is achieved by dividing time across different processes a few millisecond on process 1 and then few on process 2 then again few on 1 so on. It also handle resource management i.e. how they are shared among processes and also have mechanisms like deadlocks prevention that handle the conflicts of the resource with process. There is file manager that manage files you do not need to access hard disk directly OS do it for you so you only have to tell OS what you have to do (not how you need to do it) and OS will do it for you.

With the CLI (Command line Interface OS) we have come very far and now we have GUI(Graphical User Interface) Operating systems and many more facilities that you ever need.

for more info
https://en.wikipedia.org/wiki/Operating_system