A vmdk file extension is for VMware virtualization software. It stores virtual machine hard disk drive contents. There can be more vmdk files for virtual machines. It depends on the virtual hard drive amount specified. It also stores partition information the virtual drive can access.
Programs like VMware Workstation or Fusion open vmdk files. They open automatically when starting a virtual machine. You can also run a virtual machine by double-clicking the vmdk file. Additionally, WinMount mounts vmdk files as virtual disks. Then you can delete, copy, and modify virtual disk files.
Vmdk files store backup data. They enable running multiple operating systems on one machine. They are like physical disk drives.
Fusion and Workstation open vmdk files. The native application opening vmdk files is VMware software. Vmdk is the default VMware virtual disk format. It represents virtual machine hard disk contents. Access is possible like a physical drive with virtualization software. You can configure drive capacity with the Virtual Disk Manager utility. Earlier software used the .dsk extension. Naming can be complex.
Vmdk became an open format in 2011. It describes containers for virtual hard drives. Initially for VMware products, it is used in the Open Virtualization Format.
VMware Workstation
VirtualBox