Comprehensive car insurance covers damages from an "act of God," or events that are not caused by a car driving into something else. An "act of God" can include things like damage from a heavy tree branch falling on your car. Since you have no control over when or why a tree branch would fall on your car, this kind of accident would be covered under your comprehensive policy.
Collision insurance is a coverage that helps pay to repair or replace your car if it's damaged in an accident with another vehicle or object, such as a fence or a tree. If you're leasing or financing your car, collision coverage is typically required by the lender. If your car is paid off, collision is an optional coverage on your car insurance policy.