You are right that most new cars have backup cameras, also called rearview cameras, as standard equipment. That is because automakers are adding these systems in anticipation of a federal law that will require the equipment, most likely by 2015 or possibly sooner. The final ruling for when the law will go into effect has been delayed several times.
Each automaker takes a somewhat different approach to these systems. In some cases, the backup camera simply provides a view of what is behind the car, while in other cases there are guidelines showing "safe" and "caution" areas. The most sophisticated systems also provide some warning, such as an audible tone, when a car is moving dangerously toward an object that could result in a collision. As a result, some are more helpful than others.
All backup cameras provide far more visibility behind the car than a driver can get by turning his or her head, particularly with regard to objects that are low to the ground, such as a child's bicycle or a dog.
Pay attention to your backup camera and understand its limitations by looking at what appears in the screen in an area you are familiar with, such as your driveway. Regardless, you should always look behind your car before you enter it and look behind you before you start backing up, and use the rearview camera for guidance.