Spectacular is amaizing or something worthy of special notice while amazing is something causing wonder and amazement.
For example," The Sunset is Spectacular" "The view from the Everest is Spectacular"
"My Christmas Gift was amaizing, I would have never thought in a better thing"
But they could both be used for the same sentence. Its just more precise to know when you could use one instead of the other. Normally "Amaizing" is more commonly used...