There is often some confusion between air purifiers and air cleaners. The difference is as follows;

Air Cleaners. These work on the basic idea that contamination of separated from the air as the air passes through the air cleaner. Air cleaners usually use mechanical filters and/or electrostatic filters and/or carbon filters

Air Purifiers. These work by dosing ions and/or plasma and/or ozone into the air that passes through the purifier. This means that a large proportion of the 'air purification' is actually done in the wider air space as the additional ions/ozone/plasma, reacts with molecules within the air.

To make things a little more complicated also consider that there are combination units. These are simply a combination of contaminant separation within the unit and dosing of the air with ions/ozone/plasma. Some have an element of ionisation but are still mainly air cleaners and are therefore described as such. For more detail on the cleaning and purifying technologies then you should go to 'technology compared' section.

What do I need?

If you want to reduce the dust in the air, remove pollen and respiratory allergens then you should consider the air cleaners.

If you want to make the air seem fresher, reduce odours, control micro organisms both in the air and on surfaces and reduce airborne chemicals then you should consider air purifiers. 

If you would like both then look at combination units