This is a great question.
I spent my first couple of years in college, thinking they were the exact same thing, just the difference was one, was for a.c. and one was for D.C. Both physically and mathematically, they are not, at all; and here in lies the problem you are having, IMHO. It wasn't until I understood the two mathematically, that I would understand the differences in them physically.
a.c. is: Z = R + iX <-- This is the difference, and a huge difference.
-vs-
D.C. is: R
Without getting too complicated, you will need to understand the differences in real, and complex number's systems.
*Impedance has a complex component (reactance) plus a real component ( R).
*Resistance is simply a real component.
To directly answer your question:
When it comes to opamps, the input impedance is directly related to a feedback scenario. When open loop, the impedance is equal to R, a real component, when feedback is employed, you begin to add a complex component, a.k.a., a reactive component to it.
A really general analogy I use is: Resistance is physically constant, does not change, and it is INHERENT to the device. No matter what the input or output is, R is ALWAYS THE SAME.
Impedance: Impedance is GENERATED, it will literally change, depending on the input and output. It is not INHERENT to the physical make up of the device, and is based on frequency.
I would HIGHLY suggest learning simple RC circuits (e.g. tank circuits, and other really simple passive filters) before you get anywhere NEAR opamps and impedance as they will help you to understand impedance.
Hope this helps.