A UNIX system is a time sharing, multi tasking system. It is a modular integrated set of software that resides on and coordinates a computer system's resources. The UNIX system server as the interactive interface between you and computer system.

UNIX system is not structure as a single operating system program. Instead, it is a team of programs  that works with the operating system program to do certain jobs.                 

The UNIX system consist of a kernel the operating system program that is only a skeleton of the traditional operating  system program.

Kernel is the core of UNIX system. Every process interacts with the kernel in some way, either directly, by making a system call or indirectly by being allowed to run.