What is Audi A4's Quattro system and how does it work?

I've heard a lot about the Quattro system in Audi A4, but I don't really understand how it works. Can you explain it to me in simple terms?

The Quattro system in Audi A4 is a drivetrain technology that distributes power to all four wheels, providing superior traction and handling. It uses a center differential and electronic controls to distribute power according to the driving conditions. It's like having all-wheel drive all the time, ensuring optimum performance and stability on all road surfaces.