When you take a snapshot, you capture the state of the virtual machine settings and the virtual disk. If you are taking a memory snapshot, you also capture the memory state of the virtual machine. These states are saved to files that reside with the virtual machine's base files.

Snapshot Files

A snapshot consists of files that are stored on a supported storage device. A Take Snapshot operation creates .vmdk, -delta.vmdk, .vmsd, and .vmsn files. By default, the first and all delta disks are stored with the base .vmdk file. The .vmsd and .vmsn files are stored in the virtual machine directory.

Delta disk files
A .vmdk file to which the guest operating system can write. The delta disk represents the difference between the current state of the virtual disk and the state that existed at the time that the previous snapshot was taken. When you take a snapshot, the state of the virtual disk is preserved, the guest operating system stops writing to it, and a delta or child disk is created.

A delta disk has two files. One is a small descriptor file that contains information about the virtual disk, such as geometry and child-parent relationship information. The other one is a corresponding file that contains the raw data.

The files that make up the delta disk are called child disks or redo logs.
Flat file
A -flat.vmdk file that is one of two files that comprises the base disk. The flat disk contains the raw data for the base disk. This file does not appear as a separate file in the Datastore Browser.
Database file
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.
Memory file
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 nonmemory snapshots, you can only revert to a turned off virtual machine state. Memory snapshots take longer to create than nonmemory snapshots. The time the ESXi host takes to write the memory onto the disk depends on the amount of memory the virtual machine is configured to use.

A Take Snapshot operation creates .vmdk, -delta.vmdk, vmsd, and vmsn files.

File Description
vmname-number.vmdk and vmname-number-delta.vmdk Snapshot file that represents the difference between the current state of the virtual disk and the state that existed at the time the previous snapshot was taken.

The filename uses the following syntax, S1vm-000001.vmdk where S1vm is the name of the virtual machine and the six-digit number, 000001, is based on the files that already exist in the directory. The number does not consider the number of disks that are attached to the virtual machine.

vmname.vmsd Database of the virtual machine's snapshot information and the primary source of information for the Snapshot Manager.
vmname.Snapshotnumber.vmsn Memory state of the virtual machine at the time you take the snapshot. The filename uses the following syntax, S1vm.snapshot1.vmsn, where S1vm is the virtual machine name, and snapshot1 is the first snapshot.
Note: A .vmsn file is created each time you take a snapshot, regardless of the memory selection. A .vmsn file without memory is much smaller than one with memory.