Disk cloning is where you copy the entire contents of a hard disk onto another hard disk, so that it will boot and run as normal. Manually copying of all your data between two disks will not allow you to do this, as the new disk will be missing boot, partition and other critical information.
You may want to clone a disk in order to migrate to a solid state drive (SSD), or to upgrade to a larger disk when you run low on space. If you are migrating to an SSD which is smaller than your existing hard disk, make sure your disk imaging software supports this. Some tools will allow you to migrate to a smaller disk provided there is sufficient space to fit the data you are cloning. You may also have the option to exclude files and folders that you don’t need.