What is the major difference between a heat pump and an air conditioner?

technically speaking air conditioner is tern used for air cooling by compressor cooiling DX type but actually air conditioner can be any thing which is giving any condition to the air like velocity cooling heating filteration humidification or dehumidification all are air conditioning but normally air conditioning term is taken for cooling only.

In heat pump the compressor is capcble of reverse cycling so the cooling effect is reversed and condenser become evaporator and vice versa so it can heat in winter as well. Sometimes there is same compressor but heating is done by heating element coil (electric coil) those units can be called as heat pump but in actual their compressor are not heat pump type.

heat pump is mean that you uses energy (electrical) to move heat energy to a cooled place from colder one

air condition in winter is called a heat pump if you DX system is using a reversible valve(not normal electric heater its not called heat pump)

An air conditioner with an additional valve called as reversing valve is used. All other parts remain same.

The reversing valve reverses the refrigerant flow from the normal way. That is a condenser will become the evaporator and vice versa. This is used in winter season to warm the inside air to comfortable temperature.

heat pump is used to transfer heat fro lower temperature to higher temperature but airconditioner conditioned the air for human comfort and for industries by using heating,cooling,humidification and dehumidification

air condition cooling sysytem by comperrosor but heat pum inverse wich is heat system  using at summer to get comfortable temperature . 

Heat pumps have the ability to cool and heat fluids. On the other hand, air conditioner can only cool air, which is hte primery difference between those two HVAC systems 

While heat pumps and conventional air conditioners differ, they rely on the same principals to condition inside air temperatures. ... The main difference between the two systems is that a heat pump can reverse the process by switching the function of the evaporator and condenser coils to raise inside temperatures.

The heat pump can be reversed unlike air conditioner.

the air conditioners operate on the regular vapor compression cycle, the heat pumps operate on a reverse vapor compression cycle

Heat Pump is based on the mechanism of heating and maintaining the temperature highWhile Air conditioner is used for lowering the temperature by following Joule Thomson Effect..