There are two classifcations of backups: incremental and full.
Incremental Backup
The backup software compares the existing backup to the data that you wish to backup. If it is an exact match between the two, then no additional files will be backed up. However, if you have added or edited any file, these files will be updated in the backup, thus the incremental name. Unless all your files change very regularly incremental backups will be substantially smaller than taking full backups each time.
Full Backup
As the name suggests this involves taking a complete copy of all files each time the backup is run, even if they already exist in the most current backup.
