The AC Expansion Valve's main job is to regulate the refrigerant flow into the evaporator. At this point the refrigerant is still a compressed liquid. The Expansion Valves are small and typically block shaped.
As with everything else you will need to know the correct expansion valve for your system. Make sure you know what the OEM part number for your current valve is before replacing these. It is also important to know the sizes for the in and out hoses.