According to the boiling points of different volatile substances in the base liquor, the distillation is divided into several stages. 

Shortly after the start of distillation, volatile substances with low boiling points will first become gases and they are called heads.The volatilized gas after the heads of the liquor is often referred to as the "hearts". The remaining substances that are not easily volatile and have reached their boiling point are called tails.

In Vodka, the heads and tails are impurities and shall be removed as much as possible. But for whiskey or brandy that needs to be aged in barrels, properly retaining some of the heads and tails can sometimes bring a richer flavor to the liquor.