Cognac is a type of brandy which got the name from a town called Cognac, in France. By distilling wine and the aging the concentrated spirit (in French they call it eau de vie) in oak barrels, creates brandy. Only brandy that uses grape from the town Cognac and is produced there could be called Cognac. Just like the saying ‘all cognac is brandy, but not all brandy is cognac.
The difference between Cognac and Brandy is the types of fruits used and the distillation process. Brandy is made generally from grapes or other types of fruit by using column still whereas Cognac can be made only from 3 types of white grapes: Ugni blanc, Colombard and Folle blanche, and must be distilled twice in copper pot stills.