Okay, so let's start off with the difference between the force of friction, and the force of gravity.
Friction, is when two things rub against each-other. (Such as tectonic plates.)
Gravity, is something being held down towards the center of the earth.
(They are both caused by forces.)
Let's imagine wood is being rubbed against your hand. It isn't very comfortable, yet it moves evenly over your skin.
Now, let's use rubber. The skin on your hand is moving back and forth, along with the rubber.
Carpet. This is moving over your skin, yet causing some irritation.
With sandpaper. It is moving over your hand slightly like the rubber was, except this is trying to shape your hand, so therefore it is causing irritation.
I hope this helped in some way. May not have been the exact thing you wanted, but this should help give you an idea. (:
