What Is a Virtual Machine

What is a Virtual Machine?
A virtual machine is an instance of an operating system that is hosted on a physical machine by means of a software Virtualization platform. A virtual machine appears as a physical machine to the outside world and can be accessed as such. Because the Virtualization Platform presents physical machine resources to the Virtual Machines as generic resources, Virtual Machines do not require hardware specific drivers and can be ported across physical machines. According to VMware there are 4 primary benefits: Isolation: each VM is isolated from the others, so problems with one OS won 't necessary affect the others. Compatibility: since all physical resources are presented as a generic set of resources.
In terms of VMware, all VMs are therefore compatible with ALL x86 hardware. Encapsulation: packs a virtual machine as a compact set of files which makes it highly portable and hardware independence. It is a self-contained operating environment that behaves as if it is a separate computer. 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.
One instance of an operating system along with one or more applications running in an isolated partition within the computer. It enables different operating systems to run in the same computer at the same time as well as prevents applications from interfering with each other. This differs from a dual-boot or multiboot environment, in which the user has to choose only one OS at startup . All virtual machines run simultaneously.
Each virtual machine is like a "machine within the machine" and functions as if it owned the entire computer. The operating systems in each virtual machine partition are called "guest operating systems," and they communicate with the hardware via the virtual machine control program called a "virtual machine monitor" (VMM). The VMM "virtualizes" the hardware for each virtual machine.
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. It 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. An example is a program written in Java receives services from the Java Runtime Environment (JRE) software by issuing commands to, and receiving the expected results from, the Java software. By providing these services to the program, the Java software is acting as a "virtual machine", taking the place of the operating system or hardware for which the program would ordinarily be tailored.

References www.en.wikipedia.org/wiki/Virtual_machine www.portal.acm.org/citation
www.java.sun.com

References: www.en.wikipedia.org/wiki/Virtual_machine www.portal.acm.org/citation www.java.sun.com