Large airport (scheduled major airline service with millions of passengers/year, or major world military base).
Medium airport (scheduled regional airline service, or regular general aviation or military traffic.)
Small airport (little or no scheduled service, light general aviation traffic).
Heliport (no runways available for fixed-wing aircraft).
Seaplane base (no facilities for land aircraft).
Closed (airport, heliport, seaplane base, etc.)
More airports and/or navaids appear as you zoom in closer on the map.
An airport icon will appear green when it is the currently-selected airport, or grey when it falls outside the set of airports you're currently viewing (e.g. not in the same country, not in the list of airport's you've visited, etc.).