These Air Purifiers work based on charged plates or wires which grab the particulates as the air flows through them. These types of Air Purifiers are usually used for small rooms or areas since only a certain amount of air volume can move through them. These types of Air Purifiers usually do not have any filters which need to be replaced; instead, they have a main cleaning rod which can be removed for cleaning.

These are Air Purifiers, which output a stream of negatively charged ions, attract pollutant particles from the air. When these particles hit a surface which has an opposite charge, the particles will stick to the surface.
These types of Air Purifiers are used in the poultry industry as well as in US submarines where it is claimed to have a positive affect on those who suffer from mood disorders like depression.

Ultraviolet light or UV light is an affective way of destroying bacteria and viruses provided it is at the right intensity. UV light by itself is not sufficient for Air Purifier applications since it does not remove gasses and particles. So, UV light is usually combined with other technology such as Ionic, HEPA or Activated Carbon to provide an effective way of air purification.

These type of are Air Purifiers use Ozone which is an unstable molecule of three Oxygen atoms (O3). Since Oxygen likes to be in the format of two atoms (O2), it will give up its extra molecule and oxidizes what ever it bumps into such as particulates or chemicals. This is a very effective way of getting rid of smells and bad odors and other chemicals in the air. This is also very effective in chemical applications such as cleaning flooded homes or those with major sewage back ups.
Ozone can be dangerous to health if inhaled, so the best way to use these Air Purifiers is to turn on the Ozone and leave the space. Once the bad odor or smell is gone, you can return and turn off the Ozone generator

The HEPA (High Efficiency Particle Arresting) type filters were originally developed for the atomic energy commission and are used in industrial and commercial trades to clean rooms because of their ability to remove particles from the air. This type of filter is made out of a special grain paper or polymer sheet with very small passage for air flow. In this way only the air flows through and not much else. Since the passage of the media through fiber is so small, you would need large surface area for a reasonable amount of air flow to pass through the media. To increase the surface area the material is folded in an accordion or zigzag fashion to get the surface area in a small volume.
The HEPA Air Purifiers work very well in home for trapping particles, the only down side is that the HEPA filter needs to be changed every six months to five years depending on the amount of use, pollution and manufacturer.