Collision pays for repairs when something crashes into your car or your car collides with something -- a car, a garage, a fence, etc. Comprehensive then covers damage from everything else: hail, theft, vandalism, etc.
It is likely that if your car were damaged while parked, unused in the garage, you would be making a comprehensive claim and not a collision claim. However, odd occurrences happen daily that make people file collision claims they were not expecting.
There is always the possibility of a strange or random incident, like a hit-and-run driver running into your garage and crashing into your vehicle, in which case you would need to make a collision claim for the resulting damages.
If you share the garage with a family member and they hit your car while pulling in, this would be a collision claim as well.
Other damages or situations that could occur while your car is parked would likely fall under comprehensive coverage, the biggest being if the car was stolen. If an object in the garage fell on the vehicle or cracked its windshield, this would be a comprehensive claim.
Also, if a storm uprooted a tree and it fell on your car or flood waters leaked into your vehicle, you would make a comprehensive claim.
Car insurance policy terms and coverages differ, so read through your policy to find out what exactly your collision and comprehensive coverages actually cover and decide if this protection is worth the cost of these coverages while you are away and no one is driving your car.
Your agent should also be there to answer any questions you have about your coverages.
If your agent is not able to explain to you what to do in this situation or fulfill your car insurance needs, contact a CarInsurance.com agent. They can explain not only insurance terms and claim procedures but also tell you how they can get you low cost car insurance.