Scale By is based on the current scale of the object, and Scale doesn't take the current state into consideration.
So Scale By, will scale the current (default 100% scale) object by 100%. Making it 200%.
If you have Scale to on the same object and set it to 100% it will do nothing since it's already at 100%. If you want to make it the same size as above then Scale to 200%.
But, if you're dealing with an object that has already been reduced. Say to 50%.
Scale by 100% will scale the object by an additional 50% making it a total of 150%.
If you use Scale to 100%, it will make your 50% sized object back to it's original 100.
I guess think of Scale To as setting the scale, and as Jack said, Scale By is adding to the current scale.