With keyframe animation, you key an attribute value to a time in the Time Slider. You repeat this process with different values at different times to animate the object. When you need to animate multiple objects or attributes that interrelate, setting keyframes can quickly become a complex task.
Setting driven keys is a technique for driving the animation of one object or attribute, using another attribute. With driven keys, you create a dependent link between a pair of attributes. A change in the driver attribute then alters the value of the driven attribute. Once you establish this relationship between attributes, you don't need to animate the driven attribute separately; it occurs automatically as you animate the driver object.
This makes setting up some types of animation much more efficient. For example, you can use driven keys to make a door open when a character walks in front of it.
It's important to understand that driven keys don't actually animate the driven object, they only associate the driven attribute with its driver. The driven object reacts to changes as you manipulate its driver, but nothing happens if you playback the scene until you set keyframes on the driver object. Because driven keys do not link attributes to time, the Time Slider is not involved in a driven key relationship and displays no markers for driven keys.
See also Use Set Driven Key to link attributes.