“Dash cams are small video cameras (priced from $50 to more than $200) that can be mounted to your car’s dashboard or windshield to record what happens in front of the vehicle. More advanced models can also record interior audio and video, and rear-facing video, and even display on your rearview mirror or stream to the internet.”
This linked consumerist.com article provides five of the top reasons people buy one, and also compares some models and features: consumerist.com