In another installment of the Zen Riddles of Booze. Cognac is a brandy but not necessarily the reverse. Brandy come from the term gebrandtwien or burned wine ( Dutch/German origin).
In the middle ages wine began to be distilled in order to evade customs duties ( based on volume and further to reduce shipping volume and make it less likely to spoil in transit. When it arrived at its final destination it was watered back down to more or less its original strength. Many people found however that they liked distilled down. Brandy is an umbrella or generic term for a distilled spirit made from a fruit (as opposed to a grain). Here are some rough definitions:
Cognac is a form of grape brandy, produced under strict guidelines from certain varieties of grapes.
Armagnac is produced in a different area of France, and is quite different from Cognac.
Marc (French) is a form of unaged brandy similar to Grappa ( Italian) or Pisco (South America) all of which can run the range from lighter fluid to sublime depending on the distiller.
Aquadiente is the Portuguese version of grape brandy which may or may not be aged.
Calvados is apple an brandy produced in Northern France.