A pressure releif valve is a valve that will open when the pressure exceeds a certain level and once open will allow water to bypass the system and return the water to the tank. These are installed in very high pressure systems and will protect the pipe and equipment etc.  They will protect pumps as they will make sure that water continues to move through the pump even if a fault occurs that prevents water from running ie a valve fails to open and the pump would eventually start boiling the water in the pump causing permanent damage

A pressure regulator will regulate the pressure on the outlet of the regulator and make sure that the pressure does not exceed the pre set amount. So the regulator will make sure that any equipment connected to the outlet of teh regulator is protected however pressure will be build up on the inlet