This somewhat ambigious term can apply to any filter which consists of an electric impeller and a media chamber through which water is pumped. The traditional power filter had to be primed with water before it can start working. A somewhat inexpensive filter that can vary in efficiency depending on the size and manufacturer. The best ones in my opinion are the Aqua-Clear ones, because they utilize the entire media chamber instead of just using a cartridge. Other filter media that you can put in a power filter include sponges (which provide you with mechanical filtration and biological filtration), various medias that remove phosphates, ammonia, and other chemicals, peat (which will lower your pH), and a mixture of activated carbon and ammonia-removing granules, usually called ammo-carb or something similar.