What is the difference between a pressure reducing valve and a pressure relief valve ? and where they are used ?

Pressure reducing and pressure regulating valves are essentially the same though pressure regulating valves are considered to be more accurate in maintaining the set pressure. Also, the term pressure reducing valve is more commonly associated with liquids while pressure regulating valve or more correctly regulators are use only for gases. 

Both these types of valves control the flow of the liquid or gas through pipes and tubes. Pressure relieve valves are valves that will vent a device at a preset pressure. They are typically safety devices. The fluid is vented to an overflow area that may simply be the open environment. These types of valves are controlled by springs or are designed as rupture plates that fail at a predetermined pressure. Pressure release valves are manual valves. They can be combined with pressure relief valves. They require manual intervention and will release excess pressurized fluid in a safe controlled manner.