Different operating system used:
1. Single user, single Task Operating system
2. Multi user operating system
3. Multiprocessing operating system
4. Distributed Operating system

Multiprocessing is one of the features of operating system. It has two or more processors for a single running process. Each processor works on different parts of the same task or two or more different tasks.

Security Management features:
File access level security, System level security, Network level security.

Different server distributions in Linux OS:
Ubuntu, Linux Mint, Debian, Fedora, RedHat.

A Multi-user Operating system is a computer operating system (OS) that allows multiple users on different computers or terminals to access a single system with one OS on it.

(i) Keeping track of which portion of memory are currently being used and who is using them.
(ii) Determining which processes (or parts of processes) and data to move in and out of memory.
(iii) Allocation and de-allocation of memory blocks as needed by the program in main memory. (Garbage Collection).

iOS (formerly iPhone OS) is a mobile Operating System created and developed by Apple Inc., exclusively for its hardware. It is the Operating System that presently powers many of the company's mobile devices, including the iPhone, iPad and iPod Touch. It is the second most popular mobile Operating System globally after Android.

(i) To hide details of hardware by creating abstraction.
(ii) To allocate resources to processes.
(iii) Provide a interactive and effective interface.

The functions of OS
(i) Process Management
(ii) Device management
(iii) Memory Management
(iv) File Management
(v) Security Management
(vi) Storage Management

(i) Single-user operating system
(ii) Multi-user Operating system
(iii) Distributed Operating system
(iv) Operating system
(v) Mobile Operating system.

(i) Easy interaction between the human and computer.
(ii) Starting computer operation automatically when power in turned on (Booting).
(iii) Controlling input & output devices.
(iv) Managing the utilization of main memory.
(v) Providing security to user program.

(i) Distributed computing is a field of computer science that studies distributed systems.
(ii) A distributed system is a model in which components located on networked computers communicate and coordinate their actions by passing messages.
(iii) The components interact with each other in order to achieve a common goal.

The main use of Operating System is
1. to ensure that a computer can be used to extract what the user wants it do.
2. Easy interaction between the users and computers.
3. Starting computer operation automatically when power is turned on (Booting),
4. Controlling Input and Output Devices
5. Manage the utilization of main memory.
6. Providing security to user programs.