If you clone your internal harddrive to an external harddrive, you should be able to put that external harddrive in your system instead of the internal harddrive and boot from it.
There might be some issues if the harddrives have different sizes or are different types (e.g. SATA vs PATA).
If you make an image of your harddrive (or a partition) to a file somwhere, then you can restore the image of that harddrive (or partition) back to that same harddrive or another harddrive.
Which solution you want depends on the scenario(s) you are trying to protect yourself against, and it depends on whether you just want one clone or want multiple images.
A disk cloning program cannot make images (unless it’s also a disk imaging program), but a disk imaging can be used to clone a drive by first creating an image and then restoring it to another drive - this requires room for the image somewhere, however.
For a one-time transfer from an old harddrive to a new harddrive, a disk cloning or disk migration tool is better.