Bharat Stage IV, Bharat Stage III, Bharat Stage II and Bharat Stage I are the emission norms set by Government of India for petrol and diesel engines used to control air pollution caused by these engines, they have parity with Euro Emission Norms viz. Euro IV, Euro III, Euro II and Euro I but Indian standards are little diluted
The major Air Pollutants emitted by a gasoline engine and diesel engine are Cox (Oxides of Carbon), NOx (oxides of Nitrogen), HC (Hydro Carbons) and other PM (particulate matter)
Every Manufacturer of Vehicles and Engines must abide these emission norms set by Government of India, this means these manufactured vehicles and engines must not emit pollutants more than standards set by the Bharat Stage Emission Norms
Bharat Stage IV engines are most echo friendly engines and cause lest pollution, they are most strict pollution norms in India
Now whenever you seen the Bharat Stage Ratings of a vehicle or engine you will come to know how eco friendly that vehicle and engine is!