It measures VOCs (both natural and man-made), PM2.5, and temperature and humidity.
Coarse particles (PM10) have a diameter of between 10 micrometers and 2.5 micrometers and settle relatively quickly whereas fine (PM2.5) (0.1 to 2.5 micrometers in diameter) particles remain in suspension for longer. To put things into perspective, human hair has a diameter of 50-70 micrometers and a grain of sand has a diameter of 90 micrometers.
Sources of fine particles include all types of combustion activities (motor vehicles, power plants, wood burning, etc.) and certain industrial processes. Sources of coarse particles include crushing or grinding operations, and dust from paved or unpaved roads.
PM2.5 is made up of sulfates, nitrates, carbon, and soil.
Albuquerque reports the Air Quality Index for daily pollen and Fine/Coarse Particulates:
But these numbers are reported as "Index" values, and have to be converted to ug/m3 to compare to measured values:
Over the last week, Albuquerque has reported AQI for PM2.5 of almost 50, which should be about 15 micrograms per square meter, whereas my unit typically reports 30-50 micrograms per square meter, indoors and outdoors. It is possible that ABQ measures air quality higher from the ground than my unit, or that PM2.5 is lower during the night when I don't check it.
Here is an excellent resource for more information. Most of the graphics on this page are from this source.