They're also better on ice. Both have their advantages. Foam cores are lighter than wood cores, while wood cores give a smoother ride and a more even flex. A ski containing large amounts of metal may have a tendency to bend, but most skis don't contain enough metal to cause problems. A dampening system is anything that reduces the ski's vibration.
Less vibration means better edge grip, more control, and a smoother ride.
There are many kinds of systems, all designed to get the same result. The beefy Derbyflex plate is used by World Cup racers. Some systems are inside the ski, like K2's high-tech piezoelectric device. And most binding companies now manufacture dampening systems integral to their top-end bindings. Skiboards have grown increasingly popular over the last few years, and most major manufacturers now have some on the market.