We've heard of applications that run on Windows or Mac and so on. Thus Windows or Mac act as hosts for our computer applications.
Hence, The Operating System is an interface between the user and the hardware, and the host for the applications & software.

Thus, the applications need to access the services of the OS. This is done through what are called APIs(Application Programming Interfaces) or system calls.

The central component of the OS is the kernel. The kernel is the bridge between the applications & the hardware. Hence, It allows applications to process data at the hardware level. It can also control a process' access to the computer memory.

Sometimes, the kernel is used to limit a certain process' access to memory, a phenomenon called memory allocation/protection. This can be achieved by Segmentation & Paging.

The kernel & kernel extentions are alloted certain memory space, which, together with the user space alloted for all user apps, form the Virtual Memory. Virtual Memory is the key to multitasking kernels.