So, what it a virtual machine? A virtual machine (VM) is a software implementation of a programmable machine, where the software implementation is constrained within another computer at a higher or lower level of symbolic abstraction.

Virtual is a term that originally came from optics, to understand objects in a mirror. Objects in a mirror are reflections of an actual physical object but mirrors are not actually that object. This means that the image looks exactly like the actual object and looks to be in the same location

A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine. Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine—it cannot break out of its virtual world.

A virtual machine was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real machine". Current use includes virtual machines which have no direct correspondence to any real hardware.

System virtual machines (sometimes called hardware virtual machines) allow the sharing of the underlying physical machine resources between different virtual machines, each running its own operating system. The software layer providing the virtualization is called a virtual machine monitor or hypervisor. A hypervisor can run on bare hardware (Type 1 or native VM) or on top of an operating system (Type 2 or hosted VM).

The main advantages of VMs are:

• multiple OS environments can co-exist on the same computer, in strong isolation from each other
• the virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machine
• application provisioning, maintenance, high availability and disaster recovery

The main disadvantages of VMs are:

• a virtual machine is less efficient than a real machine when it accesses the hardware indirectly
• when multiple VMs are concurrently running on the same physical host, each VM may exhibit a varying and unstable performance (Speed of Execution, and not results) , which highly depends on the workload imposed on the system by other VMs, unless proper techniques are used for temporal isolation among virtual machines.

Multiple VMs each running their own operating system (called guest operating system) are frequently used in server consolidation, where different services that used to run on individual machines in order to avoid interference are instead run in separate VMs on the same physical machine.

The desire to run multiple operating systems was the original motivation for virtual machines, as it allowed time-sharing a single computer between several single-tasking OSes. In some respects, a system virtual machine can be considered a generalization of the concept of virtual memory that historically preceded it. IBM's CP/CMS, the first systems to allow full virtualization, implemented time sharing by providing each user with a single-user operating system, the CMS. Unlike virtual memory, a system virtual machine allowed the user to use privileged instructions in their code. This approach had certain advantages, for instance it allowed users to add input/output devices not allowed by the standard system.

The guest OSes do not have to be all the same, making it possible to run different OSes on the same computer (e.g., Microsoft Windows and Linux, or older versions of an OS in order to support software that has not yet been ported to the latest version). The use of virtual machines to support different guest OSes is becoming popular in embedded systems; a typical use is to support a real-time operating system at the same time as a high-level OS such as Linux or Windows.

Another use is to sandbox an OS that is not trusted, possibly because it is a system under development. Virtual machines have other advantages for OS development, including better debugging access and faster reboots.