Snapshot is used to take the snapshot to backup the open files. So if you are backing up system state,shadow copy,windows 2008, exchange2007 then by default it is a snapshot backup. Also for files you might have aofo selected in your job, you can check it by opening the job properties and click on advanced open file option. If that option is selected then it will do the snapshot for everything that get backed up under that selection list. So while doing the snapshot it will show you snapshot processing