Rick Savage, AFIS with Preferred First Insurance states the following:
Comprehensive covers fire, theft, glass and believe it or not, collision with an animal (mostly deer claims). Collision is damage to an object - another car, a tree, a house, a garage and it can be collision with the road (like a pothole).
This is a very simple explanation, so consult with your particular insurance company for exactly what they do cover. In my professional opinion, older cars should drop the collision coverage but keep comprehensive coverage on - mostly for the glass coverage