BackUp will make a compressed image of your drive. This is used in the event of failure of your system to re-install your system in a working condition. You can add "incremental backups" to the main backup which add only changes to your system since it was previously made thus saving time and disk space.
A drive "Clone" makes an exact copy of your drive and can be used to replace the drive it was make from, if necessary. You cannot make incremental additions. A clone is large size (same as your hard drive) and each update takes the same disk space and same time to prepare.
For normal backups, the image is better, used in conjunction with the Acronis boot disk which you prepare when you first install Acronis (if you haven't done this, do it now).