Operating System

An operating system (OS) is just a software running on computers, managing computer hardware resources, and providing common services for execution of various application software.

Kernel
In computing, the kernel is the main component of operating systems. It is a bridge between applications and hardware. Its responsibilities include managing the system's resources.

 

User Interface

Every computer operated by an individual requires a user interface. The user interface is not actually a part of the operating system. It generally runs in a separate program usually referred to as a shell, but is essential if human interaction is to be supported.

Examples of Operating System

• Linux
• Mac OS
• Windows
  

From: http://en.wikipedia.org/wiki/Operating_system