A clone is an exact copy - that means you can take out the original drive and put in the clone, and it will work immediately.
An image is the identical information but in a compressed form - this is so you can store it, prefereably out of the computer, possibly on smaller capacity media, but it can still be restored to a new drive in case of disaster.
A clone you have to make complete every single time. An image you can make complete, partial, or just an update - this makes images more convenient n the backing up process.
Remember that any backup you keep inside the system is vulnerable to some kinds of catastrophic system failure. Backups kept outside the system (caddy, USB drives, CDs and DVDs) are MUCH safer.
Remember, too, that a backup is only as good as when you do it - it soon becomes out of date.