VirtualBox - a special program to create a memory PC virtual machines. Each virtual machine can contain an arbitrary set of virtual devices and a separate operating system.
Scope of virtual machines is very wide - from performing the functions of software testing to create entire networks, which are easily scaled, load-balance and protect. VirtualBox is free, open source software.

VirtualBox is a general-purpose full virtualizer for x86 hardware. Targeted at server, desktop and embedded use, it is now the only professional-quality virtualization solution that is also Open Source Software. Modularity. VirtualBox has an extremely modular design with well-defined internal programming interfaces and a client/server design. This makes it easy to control it from several interfaces at once: for example, you can start a virtual machine in a typical virtual machine GUI and then control that machine from the command line, or possibly remotely. VirtualBox also comes with a full Software Development Kit: even though it is Open Source Software, you don't have to hack the source to write a new interface for VirtualBox.