1. OPERATING SYSTEM
An operating system is a group of computer programs that coordinates all the activities among computer hardware devices. It is the first program loaded into the computer by a boot program and remains in memory at all times.
In other words, it’s what runs your computer

2. Functions of an Operating system
1. Processor Management
duty of processor to different tasks being performed by the computer system.
2. Memory Management
allocation of main memory and other storage areas to the system programs as well as user programs and data.
3. Input/output Management
coordination and assignment of the different output and input device while one or more programs are being executed.

3. 4. File Management
the storage of file of various storage devices to another. It also allows all files to be easily changed and modified through the use of text editors or some other files manipulation routines.
5. Establishment and enforcement of a priority system. it determines and maintains the order in which jobs are to be executed in the computer system.
6. Automatic transition from job to job as directed by special control statements.
7. Interpretation of commands and instructions.
8. Coordination and assignment of compilers, assemblers, utility programs, and other software to the various user of the computer system.
9. Facilities easy communication between the computer system and the computer operator (human). It also establishes data security and integrity.

4. Components of Operating system
The kernel, which represents the operating system's basic functions such as management of memory, processes, files, main inputs/outputs and communication functionalities.
The shell, allowing communication with the operating system via a control language, letting the user control the peripherals without knowing the characteristics of the hardware used, management of physical addresses, etc.
The file system, allowing files to be recorded in a tree structure. 

5. 4 Types of Operating System
Real-Time Operating System (RTOS)
Single-User, Single Task
Single-User, Multi-Tasking
Multi-User

6. Real time Operating System (RTOS)
Used to run computers embedded in machinery, robots, scientific instruments and industrial systems. Typically, it has little user interaction capability, and no end-user utilities, since the system will be a "sealed box" when delivered for use.
An important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time every time it occurs
Examples: Wind River, QNX, Real-time Linux, Real-time Windows NT

7. Single-User, Single Task
OS designed to manage the computer so that one user can effectively do one thing at a time
The Palm OS used in many palmtop computers (PDA's) is an example of a single-user, single-task OS 59

8. Single-User, Multi-Tasking
Lets a single user interact with several programs, simultaneously
Most popular OS
Used by most all PC's and Laptops
Examples: Windows, Mac OS, Linux

9. Multi-User
A multi-user OS allows many users to take advantage of the computer's resources, simultaneously
The OS must make sure that the requirements of the various users are balanced, and that the programs
they are using each have sufficient and separate resources so that a problem with one user doesn't affect
any of the other users.
Examples: Linux, Unix, VMS and mainframe OS, such as MVS

10. Examples of Operating System
Windows 7, Vista, XP
Mac OS
Unix
Linux
Embedded Operating system

11. Advantages of Operating System
Easy to use
User friendly
Intermediate between all hardware and software of the system

12. Disadvantages of Operating System
If any problems affected in OS, you may lose all the contents which have been stored already
Unwanted user can use your own system
Compatibility