There are broadly speaking two methods of creating a backup, one is synchronisation and the other is a true backup. The two methods are subtly different and have potential consequences for users and so the choice of the appropriate method is essential.
Many backup systems use a process of file synchronisation to carry out backups. Synchronisation has its main use when a user works on more than one station to do their work and may swap use between a laptop and a networked PC for example. To synchronise you need a ‘central’ store where data is stored and a local store on one or more machines where files are worked on.
When the software synchronises it checks for the latest working copy (usually on a laptop or PC) and checks the version and date stamp of the file. It then compares this with the file in the ‘central’ store, if that file is ‘newer’ than the file in the central store it will synchronise (backup) the file to the ‘central’ store overwriting the older file.