What is the Quattro system in Audi cars and how does it work?

I've heard that Audi has a unique all-wheel drive system called Quattro, but I'm not sure what sets it apart from other AWD systems. Can you explain how Quattro works and why it's special?

Quattro is a permanent all-wheel drive system that distributes power evenly between the front and rear axles. Unlike other AWD systems that can detect and engage when needed, Quattro is always on, providing superior traction and handling in all types of driving conditions.