There are essentially 3 main types of Air Purifiers:
Ionizer air purifiers generate electrons that transform into negative ions. The negative ions then charge the particles in the air negatively. Once the particles have been negatively charged they are attracted by the positively charged collector which essentially acts like a large magnet for air contaminants. Ionizer air purifiers are cost effective to run and most are essentially noise free and quite compact. Most do not have a filter that need to be replaced but a collector that need to be cleaned regularly. In general they are not as effective as Hepa Air Purifiers and Air Sterilizers.
Hepa Air Purifiers ( High Efficiency Particulate Air Filter )
With Hepa Air Purifiers the air is sucked into the machine and through the HEPA filter by a fan. The filter collects over 99.9% of the contaminants ( allergens, pet dander, dust mites, etc) and the cleaned air is then pushed back out to recirculate around the room. Hepa air purifiers can filter huge amounts of airborne contaminants for a relatively low cost. On the downside some can be noisy and most require regular filter changes. The Blueair range of Hepa Air Purifiers are the most powerful, quiet and energy efficient Hepa Air Purifiers you can buy. Many Hepa Air Purifiers now come with an Ionizer built in to offer the best of both worlds in terms of freshening the air and actually trapping the particles.
Air Sterilizers like the Airfree models, clean the air to the highest possible standard and even destroy viruses and bacteria. They are also completely silent in operation and have no filters to replace. Air sterilisers work well if you are concentrating your efforts on one specific room and want the cleanest air possible.