I do believe he asked how wet/dry's work. A wet dry filter is box that has filter media inside it which the water trickes over then into the sump part where your pump returns it to the tank. The water comes from the tank either through holes that have been drilled into your tank, or from a continous syphon overflow box, either way they are both plumbed into the wet.dry filter. It is very important to know that when selecting a pump that you know how much water your overflow box is rated for, or the pre-drilled holes are rated in gallons per hour, you dont want to be trying to pump more water into your tank than what can come out.
As for heating, you could probably go with two 350 watt heaters, which you could place right into the sump of the wet/dry filter that way you wouldnt have to see them in your tank. Your sump should always have a high enough water level to keep your heaters submerged because your pump also, of course, needs to stay underwater.
Also with a wet.dry filter the water level never changes in your tank, it changes in your filter. Which is nice cause you dont see the water line.