What is a Shunt and where is it used ?

A shunt is defined as a low resistance path across a circuit.  Shunts are used commonly in the measurement of electric current.  

For instance, if you have an ammeter which can measure current only up to 5A and you want to measure 10A.  You can connect a resistance in parallel to the ammeter.  If the value of the resistance in parallel is the same as the internal resistance of the ammeter, the current will be evenly divided between the instrument and the shunt resistance.

Thus when the instrument indicates 5A, the value of the current in the circuit is 10A.  Thus, the shunt resistance is used in parallel to the main ammeter to increase its range.