H bridge circuit

 

H bridge  is an electronic circuit that enables a voltage to be applied across a load in either direction.
These circuits are often used in robotics to allow DC motors to run forwards, backwards and to give electric break(short circuit condition). H bridges are available as integrated circuits, or can be built from discrete components as figure 1.
motor driver circuit using relay(6v) and diode(1IN4007)
and this relay circuit is called H bridge circuit and diodes are added to make easy controlling of robots.
so by 6v input signal its control high output voltage.
 

                                                                    schematics for H bridge(Fig. 1) 

In this circuit there are 3 power supply connection(including two ground which are different)
4 motor output ( which control 2 motors)
and directional input to get more user friendly output control.