If you take a look the How Stuff Works article on washing machines, you'll learn that damping systems use friction to absorb some of the force from vibrations. A damping system in a building is much larger and is also designed to absorb the violent shocks of an earthquake. The size of the dampers depend on the size of the building. This is an uncontrolled damper, which requires no input power to operate. They are simple and generally low in cost but unable to adapt to changing needs.
Active dampers are force generators that actively push on the structure to counteract a disturbance.
They are fully controllable and require a great deal of power. Rather than push on the structure they counteract motion with a controlled resistive force to reduce motion. They are fully controllable yet require little input power. Unlike active devices they do not have the potential to go out of control and destabilize the structure. MR fluid dampers are semi-active devices that change their damping level by varying the amount of current supplied to an internal electromagnet that controls the flow of MR fluid.