Backup applications for the Virtualized environment back up the Virtual Machine at the image level and are not aware of what is going on inside the installed guest operating system.
So before you back up your Virtual Machines you should ensure they are quiesced so they are in a consistent state to be backed up.
Quiesce is to pause or alter a device or application to achieve a consistent state, usually in preparation for a backup.
The quiesce operation is handled inside the guest OS, and for Windows VMs, the Volume Shadow Copy Service (VSS) handles this. If you don’t quiesce them, you risk having data that is not in a state to be restored properly.
The backup operation is taken care of your backup application which is at the virtualization layer and not inside the guest OS so it requires another application to inform the guest OS to quiesce the VM.
In the case of VMware vSphere, that application is VMware Tools, which tells the VSS service to quiesce the guest OS. For VMs running Linux operating systems with no native services like VSS, VMware Tools also provides a special vmsync driver that can provide the same functionality as VSS. This makes it very important that VMware Tools be installed and kept up to date on all your VMs. There are also instances where VMware Tools may not support certain guest OS versions, so always check to see if your version is supported by the application.
Some backup application vendors like Nimesa ( which i am working on) supply their own special agent that will handle the quiesce process.