Virtualization

Virtualization:Industry moving from concept of buying to leasing
    Software as a Service  ( SaaS)
    Platform as a Service  (PaaS)
    Hardware as a Service  ( HaaS)

Virtualization is the creation of a virtual (rather than actual) version of something, such as an operating system,  a server, a storage device or network resources.
Allows to create multiple virtual machines on a single physical server
Allows to run multiple operating System environment in a single Physical server.
Better utilization of Hardware resources  -  CPU, Memory
Reduces the need for huge no of  Physical servers

There are several different forms of virtualization that need to be understood as a basis for making the right technology choice.
 * Network virtualization is a method of combining the available resources in a network by splitting up the available bandwidth into channels, each of which is independent from the others, and each of which can be assigned (or reassigned) to a particular server or device in real time. The idea is that virtualization disguises the true complexity of the network by separating it into manageable parts, much like your partitioned hard drive makes it easier to manage your files.
    * Storage virtualization is the pooling of physical storage from multiple network storage devices into what appears to be a single storage device that is managed from a central console. Storage virtualization is commonly used in storage area networks (SANs).
    * Server virtualization is the masking of server resources (including the number and identity of individual physical servers, processors, and operating systems) from server users. The intention is to spare the user from having to understand and manage complicated details of server resources while increasing resource sharing and utilization and maintaining the capacity to expand later.
    * Server Hardware Virtualization. Also known as a hypervisor, Server Hardware Virtualization runs a very lightweight core operating system. The hypervisor can host independent virtual machines (VMs). This form of virtualization requires hardware that has embedded virtualization awareness capabilities. Since the hypervisor is very lightweight, there is little overhead in the system, which allows for more scalability in the virtual machines.
    * Server Software Virtualization: An operating system, such as Windows Server® 2003 or Windows Server 2008, runs an application that is able to host virtual machines. Each virtual machine runs a completely separate operating system and application set.
    * Presentation Virtualization: Centralized systems host multiple user sessions, and all processing is done on those host systems. The user sessions are isolated from each other. Only the presentation information, such as keyboard and mouse inputs, and video updates are sent between the client and the host system. The client can be a full Windows-based workstation or a Windows-based terminal device.
    * Application Virtualization: An application is isolated from the underlying operating system by means of wrapper software that encapsulates it. This allows multiple applications that may have conflicting dynamic link libraries (DLLs) or other incompatibilities to run on the same machine without affecting each other.
    * Desktop Virtualization: This is similar to Server Software Virtualization, but it runs on client systems such as Windows Vista®. The client operating system runs a virtualization application that hosts virtual machines. This is often used when a specific person needs to run one or a limited number of legacy applications on a legacy operating system.
Virtualization products
Linux
Linux virtualization users have two free, open source hypervisors to choose from: Xen and KVM.
VMware
VMware workstation
VMware Fusion ( MAC os)
VMware server  ( GSX Server)
VMware ESX Server 4.0/4.0i & Vsphere
Microsoft
Microsoft Virtual PC
Microsoft Virtual Server
Hyper-V  (This feature available only on Microsoft Windows Server 2008 )
Citrix
XEN Server
Virtualization environment
Host Machine -  Physical server/Machine where Virtualization software deployed
Virtualization software  :  Software that allows to create, manage virtual machines & Virtual environment
Guest operating system :  Operating system installed into virtual machine 
Virtual Machine or VM :A machine is made up of Software files, provides Virtual hardware created by Virtualization software.A Virtual Machine will have Virtual Devices like Processor, Disk, Ethernet, Motherboard, VGA, etc..

properties of user in linux

To check the user properties grep for a user name. 
root# grep user-name | /etc/passwd
user-name:x:500:500: :/home/user-name:/bin/bash # this line contains 7 information of a user.

{user-name}:{x}:{500}:{500}:{ }:{/home/user-name}:{/bin/bash} # I have divided each part with a bracket.

1.{user-name}                    = User name 
2.{x}                                   =Encrypted password
3.{500}                               =UID
4.{500}                               =GID
5.{ }                                    =Comment
6.{/home/user-name}         =User's default or home directory
7.{/bin/bash}                      =Default Shell of Linux

Users in Linux

In Linux platform their are 2 types of user such as System User and Normal User
System User : The user which are created default by Operating System are called system users.The function of this user to access or manange system services which cannot be utilized by normal user. The system services are Ex:shutdown,restart and application service like mail services and apache services etc..
The system users can also be recognized with the help of an UID(Unique Identifier) which starts from 0(zero) and  reserved up-to 499 by default.

Normal User: The user which are created manually by an administrator or by a system user to access system objects and resources are called normal users. Normal user can be recognized with the help of an UID(Unique Identifier) which starts from 500 and reserved up-to 60,000 by default.