1. An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. 

Examples:

Windows: Microsoft Windows has existed in one form or another since 1985 created by Bill Gates, and it remains the most popular operating system for home and office computers. Its latest versions, including Windows 10, are also used on some tablets, and the OS is used on some web and number-crunching server computers as well. Computers from a wide variety of manufacturers can use Windows.

Apple iOS: Apple's iOS is one of the most popular smartphone operating systems by Steve Jobs, second only to Android. It runs on Apple hardware, including iPhones, iPad tablets and iPod Touch media players.

Signature features of iOS include the App Store where users buy apps and download free software, an emphasis on security including strong encryption to limit what unauthorized users can extract from the phone, and a simple, streamlined interface with minimal hardware buttons.

Android OS: Android is the most popular operating system in the world judging by the number of devices installed. Largely developed by Google, it's chiefly used on smartphones and tablets. Unlike iOS, it can be used on devices made by a variety of different manufacturers, and those makers can tweak parts of its interface to suit their own needs.

2. TYPES OF OS.

GUI: A GUI (graphical user interface) is a system of interactive visual components for computer software. A GUI displays objects that convey information, and represent actions that can be taken by the user. The objects change color, size, or visibility when the user interacts with them.

Disadvantages: They need significantly more memory (RAM) to run than other interface types

They use more processing power than other types of interface

Advantages: It is easy to explore and find your way around the system using a WIMP/ GUI interface

You do not have to learn complicated commands

CLI: A command line interface (CLI) is a text-based user interface (UI) used to view and manage computer files.

Advantages

If the user knows the correct commands then this type of interface can be much faster than any other type of interface.

It needs less RAM.

Does not use as much processing time.

Disadvantages

For someone who has never used a CLI, it can be very confusing.

Commands have to be very precise. If you make an error the command will fail.

If you mis-type an instruction you will have to start from scratch again.

3. CLASSIFFICATIONS OF OS.

i) Multiuser OS:

In a multiuser OS, more than one user can use the same system at a same time through the multi I/O terminal or through the network.
For example: windows, Linux, Mac, etc.
A multiuser OS uses timesharing to support multiple users.

ii)  Multiprocessing OS:

A multiprocessing OS can support the execution of multiple processes at the same time. It uses multiple number of CPU. It is expensive in cost however, the processing speed will be faster. It is complex in its execution. Operating system like Unix, 64 bit edition of windows, server edition of windows, etc. are multiprocessing.

iii)  Multiprogramming OS:

In a multiprogramming OS more than one programs can be used at the same time. It may or may not be multiprocessing. In a single CPU system, multiple program are executed one after another by dividing the CPU into small time slice.
Example: Windows, Mac, Linux, etc.

iv) Multitasking OS:

In a multitasking system more than one task can be performed at the same time but they are executed one after another through a single CPU by time sharing. For example: Windows, Linux, Mac, Unix, etc.
Multitasking OS are of two types:
a) Pre-empetive multitasking
b) Co-operative multitasking
In the pre-empetive multitasking, the OS allows CPU times slice to each program. After each time slice, CPU executes another task. Example: Windows XP.
In co-operative multitasking a task can control CPU as long as it requires. However, it will free CPU to execute another program if it doesn’t require CPU. Example: windows 3.x, MultiFinder, etc.

FUNCTIONS OF OS.

1. Security –
   The operating system uses password protection to protect user data and similar other techniques. It also prevents unauthorized access to programs and user data.

2. Control over system performance –
   Monitors overall system health to help improve performance. Records the response time between service requests and system response to have a complete view of the system health. This can help improve performance by providing important information needed to troubleshoot problems.

3. Job accounting –
   Operating system Keeps track of time and resources used by various tasks and users, this information can be used to track resource usage for a particular user or group of user.

4. Error detecting aids –
   Operating system constantly monitors the system to detect errors and avoid the malfunctioning of computer system.
5. Coordination between other software and users –
   Operating systems also coordinate and assign interpreters, compilers, assemblers and other software to the various users of the computer systems.
6. Memory Management –
   The operating system manages the Primary Memory or Main Memory. Main memory is made up of a large array of bytes or words where each byte or word is assigned a certain address. Main memory is a fast storage and it can be accessed directly by the CPU. For a program to be executed, it should be first loaded in the main memory. An Operating System performs the following activities for memory management:

It keeps tracks of primary memory, i.e., which bytes of memory are used by which user program. The memory addresses that have already been allocated and the memory addresses of the memory that has not yet been used. In multi programming, the OS decides the order in which process are granted access to memory, and for how long. It allocates the memory to a process when the process requests it and de-allocates the memory when the process has terminated or is performing an I/O operation.