A Virtual Hard Disk (VHD) is a virtual hard disk file format, meaning it can contain what is found on a physical hard disk drive, such as disk partitions and a file system, which in turn can contain files and folders. It is typically used as the hard disk of a virtual machine. The format was created by Connectix, which was later acquired by Microsoft, for what is now known as Microsift Virtual PC. Since June 2005, Microsoft has made the VHD Image Format Specification available to third parties under the Microsoft Open Specification Promise 

    A Virtual Hard Disk allows multiple operating systems to reside on a single host machine. This method enables developers to test software on different operating systems without the cost or hassle of installing a second hard disk or creating a separate partition on a single hard disk. The ability to directly modify a virtual machine’s hard disk from a host server supports many applications, including:

•       Moving files between a VHD and the host file system
•       Backup and recovery
•       Antivirus and security
•       Image management and patching
•       Disk conversion (physical to virtual, and so on)
•       Life-cycle management and provisioning