A car phone is a cellular phone designed specifically to be mounted in a car, and usually powered by the car battery.
Ideally, it is only meant for emergencies: For example, if you are involved in a car accident and a regular phone is unavailable, the car phone can be a real life saver.
In real life, unfortunately, many people use their car phone as a status symbol. I used to have a friend who loved to show off all of his technology gadgets. Whenever he would drive his car and have a passenger, he would call someone on his car phone as soon as he started moving the car.
I always felt uneasy about that. For one, it was rude: He invites a friend for a ride, then starts a conversation with someone else.
More importantly, it is very dangerous: It distracts the driver. That way, a device meant to be a life saver in emergencies actually becomes an accident waiting to happen.