By default, unless you change that somehow, iTunes keeps at least one running backup for each device that you decide to backup to the computer. These backups are incremental, every time you connect a device it backs up what has changed since the last sync. In addition, when you restore a device via iTunes, iTunes offers you an extra backup. This is sort of the last device snapshot before it gets erased in the restore process. That restore backup should show a time stamp in its name for easy identification.