Basic Question -- Unclear about ..

Question about what and how is data stored ....

I'm used to backing up an entire hard disk with all partitions and files using True Image Home, then making Differential or Incremental backups until time to make an entirely new full backup

My question is, does the on-line backup do the same thing, i.e., backup only the modified files, or does it make a new backup of the entire hard disk at each period selected; daily, weekly, monthly, etc?

Wouldn't that be rather time consuming and fill up available space rather quickly if new backups are made frequently?