The refrigerant is the carrier of heat: it boils (evaporates) at a low temperature at low pressure and condenses at a high temperature at high pressure; this is how it can give the heat it took from a 0 °C room to 40 °C outside air. "Freon" is actually a brand name; in everyday speech it is used for all synthetic refrigerants. Refrigerants are referred to by the letter R and a number (R404A, R134a).
By type: HFCs (R404A, R134a, R410A) are harmless to the ozone layer but have a high greenhouse effect; HFO blends (R449A, R448A, R454C) are the lower-GWP new generation; natural refrigerants — propane (R290), carbon dioxide (R744) and ammonia (R717) — have near-zero GWP but their own safety requirements such as flammability, high pressure or toxicity.
The choice is made considering application temperature (R404A/R449A at low temperature, R134a/R513A at medium temperature), legal status (the F-gas regulation is phasing out high-GWP refrigerants), safety class, equipment compatibility and efficiency. Every refrigerant has its own pressure–temperature relationship; our refrigerant P–T chart lists the saturation values, GWP and safety class of 12 refrigerants.