Demonetisation is the act of removing a currency unit of its status as legal tender. Demonetisation is necessary whenever there is a change of national currency. The old unit of currency must be removed and replaced with a new currency unit. It is usually done to combat inflation or corruption.
Recently, India demonetised the Rupees 500 and 1000 notes- approximately 86% of the currency in circulation.
The effects of such a decision are analysed in the Guardian & Financial Times:
Financial Times- India’s demonetisation in four charts
Similarly, Venezuela too decreed on 11 December that the 100-bolivar bill would be pulled from circulation in 72 hours.