The Dryer or Accumulator (depending on the air con system) has several functions, the main being to protect the compressor from damage and erosion.  This is done by absorbing dampness (moisture), removing small metallic debris found in the system and storing excess liquid refrigerant.

A Receiver dryer will be found in an air con system which uses an expansion valve.  The Accumulator is used in systems with an orifice tube, both being found in the engine compartment.