Let's use the aftermath of Superstorm Sandy as an example to illustrate the differences between collision and comprehensive. Within that storm, let's consider two events that might have happened: 1) a heavy tree branch fell on your car, or 2) you swerved to avoid a falling tree branch and wound up crashing into a tree. In the first event, you had no control over when or why a tree branch would fall on your car. This kind of accident would get reimbursed under your comprehensive policy. In the second situation, you were driving the car and ultimately swerved into the tree, which makes it a collision, and collision insurance therefore pays for the damages. Events like the hypothetical ones stated above are why it's important to differentiate between the two types of coverage.
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.