An Operating System manages computer’s resources to provide a controlled environment where user applications perform their activities, without (hopefully) interfering in each others operations. Resources include CPU Processing time, Memory and computer peripherals. Operating systems are complicated software which makes it possible for normal people to easily use computers.
One of the main reasons for widespread usage of computers in recent times is the availability of wide range of applications that run on them. All these useful applications uses the platform provided by the operating system. Developer don’t need to completely understand the hardware organization of the computer on which their applications run. Writing applications in the absence of operating system would be a very cumbersome task.
I believe MS DOS has done more to spread computing than any other operating system. It was a simple OS to use and develop upon. This was probably its biggest strengths. Microsoft used its success to move the masses to their Windows line of Operating Systems. Linux is one another OS which has become quite famous since 1991 when Linus Torvalds released it to general public for free! But before all these, Unix had established its self as a model for modern operating systems.
Core component of an operating system is called Kernel. Kernel gels together different parts of an OS, effectively managing the resources like CPU, memory and Input/Output (IO) devices. A well designed kernel provides fault-tolerence, security and deterministic (real time) properties to a computer system.