HEPA is short for High-Efficiency Particulate Air filters. HEPA filters have hundreds of square feet of filter paper folded into a couple of square feet of space. They remove very small particles, including some viruses. Because they are several inches thick, they require duct modifications. Electronic air cleaners appear in many modern forced air heating systems.
Some units also use a panel-type prefilter and a carbon filter.
Plates or wires in the unit have an electrical charge. Particles in the air passing over the charged surfaces pick up the charge. Oppositely charged plates further downstream collect the particles. These units generate a small amount of ozone, which may cause health concerns for some people. Carbon filters downstream remove some of the ozone.