A snapshot consists of files that are stored on a supported storage device. A Take Snapshot operation creates .vmdk, -flat.vmdk, .vmsd, and .vmsn files. By default, the first and all subsequent snapshots are stored with the virtual machine base files.
A delta disk has two files, including a descriptor file that is small and contains information about the virtual disk, such as geometry and child-parent relationship information, and a corresponding file that contains the raw data.
NOTE If you are looking at a datastore with the Datastore Browser in the vSphere Client, you see only one entry to represent both files.
In above figure you can see 000002–delta.vmdk is bigger in size compared to 000002.vmdk file
The files that make up the delta disk are referred to as child disks or redo logs.A child disk is a sparse disk. Sparse disks use the copy-on-write mechanism, in which the virtual disk contains no data in places, until copied there by a write operation. This optimization saves storage space. A grain is the unit of measure in which the sparse disk uses the copy-on-write mechanism. Each grain is ablock of sectors that contain virtual disk data. The default size is 128 sectors or 64 KB.
Database file (vmsd)
A .vmsd file that contains the virtual machine’s snapshot information and is the primary source of information for the Snapshot Manager. This file contains line entries, which define the relationships between snapshots and between child disks for each snapshot.
.vmsd file contents grabbed from putty
A .vmsn file that includes the active state of the virtual machine. Capturing the memory state of the virtual machine lets you revert to a turned on virtual machine state. With non-memory snapshots, you can only revert to a turned off virtual machine state. Memory snapshots take longer to create than non-memory snapshots. The time the ESX host takes to write the memory on to the disk is relative to the amount of memory the virtual machine is configured to use.
In above figure two snapshots are created.