The core differences between the masks are the mask style and headgear.
The AirFit F30 and AirFit F20 are ResMed Full Face Masks while AirFit N20 and AirFit N20 Classic are ResMed Nasal Masks and the AirFit P10 is a ResMed Pillow Mask. The core differences between the mask is the mask style and headgear.
The main difference between the AirFit F30 and AirFit F20 is the fit. The F30 Mask is designed with nothing on top of your nose so there's no risk of red marks or discomfort on the bridge of your nose. The AirFit F20 is designed to cover the mouth and nose.
As for the AirFit N20 and AirFit N20 Classic mask, the difference is the fit. The N20 Classic Mask is designed with forehead support, while the N20 Mask is designed without the forehead support. It all depends on your personal preference.
The AirFit P10 Pillow Mask is designed to fit the base of your nose, so that you can move freely while you sleep.
Full Face Masks which covers the mouth and nose are for mouth breathers and those with nasal obstruction. The Nasal Masks which cover the nose are designed to appeal to those who do not like the feeling of the air pressure directly into their nares but do not need a full face mask, or prefer the extra stability of a nasal mask over the nasal pillows style. The Nasal pillows masks rest at the entrance of your nostrils.