Originally developed specifically for use in various VMware virtual appliances, a VMware Virtual Machine Disk, also known as a VMDK or, in some contexts, a virtual disk, is a file that holds all of the data necessary to serve as an independent virtual machine. While it was initially created with VMware Workstation, VMware Fusion, and VMware Player in mind, it has since adopted the open source format to ensure compatibility with other platforms.

What is a Virtual Machine?
In simplest terms, a virtual machine provides all of the architecture and functionality of a full computer system – including either its hardware, software, or both – without the need for the physical components. It's a means of using one system to mimic the functionality of another, and it's done for a variety of reasons.

• Running various operating systems, such as Windows and Linux, without switching to another workstation or machine
• Operating outdated, legacy software
• Implementing disaster recovery and data redundancy within a virtualized environment
• Developing custom or proprietary software for another platform

There are other uses, too, such as cloning systems, dealing with malware, and more. When used by an experienced VM administrator, a virtual machine can help track and organize assets, boost productivity, and even reduce overhead.

VMDK on Other Platforms
As mentioned, the VMDK extension is compatible with other platforms, too. Some of the most popular third-party platforms that are currently supporting VMDK include:

• VirtualBox
• Parallels Desktop for Mac
• Norton Ghost
• Sun xVM
• SUSE Studio
• Paragon Hard Disk Manager

Making the VMDK extension available on other platforms adds an extra level of versatility to VMware and VMware appliances that wasn't available before its introduction into open source. Since then, VMDK has also become one of the primary extensions used in the Open Virtualization Format standard.

VMDSK Data Storage and Provisioning
Initially, the VMDK file extension was limited to 2TB in capacity. However, the introduction of VMware vSphere 5.5 in September 2013 effectively increased this limit to 62TB on all platforms. Additionally, VMDK virtual disks can be provisioned using one of three different methods:

• Thin: The initial VMDK image is created in the form of a sparse file, and any necessary block allocation or zeroing is completed upon first access.
• Zeroed Thick: All blocks are allocated upon creation, but they are not zeroed until they're accessed for the first time.
• Eager Zeroed Thick: All blocks are allocated and zeroed upon creation and initial provisioning.

Most users utilize thin provisioning to save as much storage capacity as possible. Those who want increased fault tolerance will generally opt for one of the other two provisioning methods.

VMDSK in Modern Computing
While VMDSK usage was once relegated strictly to the VMware family of products, the VMDSK file extension was eventually made available to the open source community. As expected, this caused it's usage to skyrocket and, as a result, it's currently one of the most popular virtual disk extensions in use today.

You may read more about VMDK disk image in Wikipedia: VMDK.