Hyper-V: Differencing disks

From Technet Site

Differencing disks, a way to save space, but still deploy the same amount of virtual machines.  This solution allows you to create one parent image with a base Windows installation (including common patches/updates and software) and then setup all other VM’s based on that one parent image.  So, you’d create one parent image, 30GB in size, with W2k8R2 plus all the latest updates, and then create child images for each other VM, only the delta’s of those VM’s (changes between the parent and child) are stored (differencing disks).  This is especially useful if space is at a premium, or storage space is expensive.