If you want to copy your entire hard disk (or partition), including unrecognized or unused parts of the disk, some disk imaging tools offer a sector by sector backup option. This typically won’t be needed, but in some cases if you have corrupted system files, hidden partitions or an unrecognized filesystem, you may want to do this.
A sector by sector backup is best thought of as a unsophisticated brute force copy, where it copies the full hard disk (bit by bit), regardless or whether it recognizes any data being there or not. This type of backup will normally take up a lot more space than a normal system image backup, since it will not use compression, and will copy unused or empty space.