Cognac is a brandy made from grapes that are very acidic. After fermenting into wine, it is distilled twice within strict parameters.
Cognac is a blend of many vintages. Its age is determined only by the amount of time spend in the oak cask.
VS means that the average age of the blend is 5 to 9 years. VO, VSOP means the average age of the blend is from 12 to 20 years.
XO, Napoleon, VVSOP, Cordon bleu, Vielle reserve and others designate that a large portion of the blend of cognac contained has been aged for 20, 30, 40 years or even longer.