Air Purifiers

What types of pollutants can an air purifier remove?
What is a HEPA filter? Does it have anything to do with the certification process of the ENERGY STAR certified air purifiers?
What air purifier technologies do ENERGY STAR certified room air purifiers use?
Is there a difference between room air purifiers and room air cleaners?
How is the performance of an air purifier measured? What is the CADR-to-Watt ratio?