A powered vent is just as it sounds. It uses some type of power source to operate a fan to vent the hot air out of the attic. Often the power source is solar but can be from another type of electrical power supply as well. A powered vent can give your attic a boost in reducing moisture buildup and also lowering energy costs. Powered vents are often a great solution in areas of the country that experience severe temperature changes during the winter months rendering traditional passive vents systems ineffective. Powered vents can sometimes move enough air that one vent is all you need. Keep in mind that unless you use a solar powered vent, that there will be some small annual operational costs on top of the cost of vent installation. Like any vent system, any exhaust based vent system needs to be balanced with an intake system that matches the airflow. One of the drawbacks of the powered system is the inability to balance the system easily.
Like all vent systems, the consultation of an experienced professional is important to help you determine the best solution for your home.