A VMDK, or Virtual Machine Disk, is a file format used by virtualization software like VMware to store the contents of a virtual machine's hard disk. It allows multiple virtual machines to run on a single physical machine by simulating a complete computer environment, including the operating system and applications. VMDK files can be used for various purposes, such as creating backups, transferring virtual machines between hosts, or expanding storage. They can also be split into multiple files for easier management and can be converted to other formats, making them versatile in cloud computing and data center environments.