A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine.

An image of a virtual machine is (in simple words) a copy of the VM, which may contain an OS, data files, and applications (just like your personal computer).

An image is a virtual hard disk (.vhd) file that is used as a template for creating a virtual machine. An image is a template because it doesn’t have the specific settings that a configured virtual machine has, such as the computer name and user account settings. If you want to create multiple virtual machines that are set up the same way, you can capture an image of a configured virtual machine and use that image as a template.

For more information refer to:

http://www.windowsazure.com/en-us/documentation/articles/virtual-machines-capture-image-windows-server/

A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine.

An image of a virtual machine is (in simple words) a copy of the VM, which may contain an OS, data files, and applications (just like your personal computer).

It's like a computer built inside your computer.

Hi Kaled!

I don't know from which context you took that sentence, but in general context, as Atta ur Reham Khan pointed, they are referring to the file containing the OS, data files and applications that will be used when your VM runs. Imagine that your VM is your computer with no OS installed. When you start the VM, it will get the OS to run from that image file. Unfortunatelly, the file may vary according to the hypervisor/service being used. This is a concern when you migrate services from one IAAS provider to another. If you configure your OS, applications, data and so on and then decide to change your provider, you will want to use the same image when you migrate. But this is subject for another discussion...

"A hard disk image is interpreted by a Virtual Machine Monitor as a system hard disk drive. [...] In terms of naming, a hard disk image for a certain Virtual Machine monitor has a specific file type extension, e.g., .vmdk for VMware VMDK, .vhd for Xen and Microsoft Hyper-V, .vdi for Oracle VM VirtualBox, etc."

Source: http://en.wikipedia.org/wiki/Virtual_disk_image#Virtualization

We need to consider two things: (1) Installed OS along with files stored in a single file which we can refer as Virtual Hard Disk (2) Currently running OS along with currently running processes (i.e., in memory image, it is similar to a file that is created when we do hibernate in Microsoft Windows) which can be referred as image of virtual machine.

An image of a virtual machine is used to transfer the virtual machine (running instance) from one hosting computer (physical) to another hosting computer.

There are several definitions of "VM," depending on what you are looking for.

1. As Harshadkumar pointed out, one view of a virtual machine is simply the 'snapshot' of a running machine. Unlike a hibernated machine, the capture is literally between CPU cycles. Take that image and restart it and it will continue on blithely as if from suspended animation (but probably requiring reauthenticating to attach to disk shares and whatnot). In other words, this is an image of a running machine. To use Amazon's phrasing, it's called a "snapshot."

2. Soha points out that it might include a hard disk. That's generally NOT the case, since storage is usually considered a separate resource and is stored differently. However he is correct in that the image is presumed to live on a virtual boot image, which has a certain size. Amazon has the ability to create volumes, which are images of a storage space that can be reinstantiated just like a for use by multiple VM instances.

3. The strategically useful definition of a VM (called an AMI in Amazon's cloud) is a ready-to-boot server image. An instance is not, like the above two, tied to a specific architecture or machine: it can be set to run on virtual instances of multiple types, not the same core/memory required by the above. An example would be using VMs of a application portal to create a dynamically sized solution behind a load balancer. The VM doesn't contain live information; it's like a server just needing bootup to function in its role.

Hope this clarifies things.

In my experience, a VM OS provides to each running guest OS a set of virtual registers, processor memory, peripheral devices and maintains the states of those virtual resources, so that each time the guest OS is dispatched by the VM OS, it is presented with those resources in the states last seen by the guest OS. In this sense the 'image' of the virtual machine is the state of computer resources, registers, memory, peripheral devices, etc., so that the memory has the values expected by the guest OS and outstanding I/Os can be expected to be serviced through subsequent virtual interrupts, etc. - just as if the guest OS were running on a dedicated computer system.

The 'image' is referred to as the virtualized computer system defined for the guest OS - hypothetically, at least, a virtual machine image could even include HW, firmware or SW emulation of a completely different processor architecture from the host computer: a virtual machine image might be defined such that a PC-DOS system might operate just as though it were running on an ('ancient') Intel 8080 processor - in this case that specific VM image might be described as an I8080 image system...

Please see our paper indicated in the link below to understand better this subject:

https://www.researchgate.net/publication/220144318_Toward_an_architecture_for_monitoring_private_clouds?ev=prf_pub

ABSTRACT Cloud computing is rapidly emerging as a new model for service delivery, including for telecommunications services (cloud telephony). Although many solutions are now available, cloud management and monitoring technology has not kept pace, partially because of the lack of open source solutions. To address this limitation, this article describes our experience with a private cloud, discusses the design and implementation of PCMONS (Private Cloud Monitoring System) and its application via a case study for the proposed architecture. An important finding of this article is that is possible to deploy a private cloud within the organization using only open- source solutions and integrating with traditional tools like Nagios. However, there are significant development work to be done while integrating these tools. With PCMONS we took first steps towards this goal, opening paths for new

development opportunities as well making PCMONS itself an open-source tool.

Article Toward an Architecture for Monitoring Private Clouds