In the same sense that Scotch is a form of whisky, Cognac is a form of brandy.
By and large there is no difference between brandy and cognac as much as there is a difference between Cognac and other types of brandy spirits (like Armagnac, for instance). Specifically, all cognac comes from the Cognac region of Southwest France, home to six distinct terroirs and the white grapes therein.
What is Brandy?
Any spirit that is distilled from fruit-based wine, though in general it refers to grape wines. Brandy can be made from any fruit wine such as pear, plum, or apple (Calvados) as long as it is labelled with the fruit it is made from. The taste of brandy varies depending on the fruit it is made from and its age, but generally they are sweeter than whiskey and taste of flowers, fresh and dried fruit, and citrus zest. Brandy must be aged in barrels and has different age designations for each level of quality.
While older brandies are certainly enjoyable sipped straight, in many cases due to the flavour profile of brandy, it also pairs very well with citrus. When using Brandy cooking and making desserts, on the Very Special brandy that is aged for at least 2 years should be used.
What is Cognac?
All Cognacs are brandies but not all brandies are Cognacs. In order for a brandy to be a Cognac, it must follow very strict regulations which are governed by the AOC (Appellation d’Origine Controlée) in France.
According to the guidelines, All Cognac must be grown and produced in the Cognac region in the southwest of France. It can be made only from the Ugni Blanc, Colombard, or Folle Blanche grape varieties. It must be distilled twice in copper Alembic stills. The resulting distillates, known as eaux de vie or waters of life, must then be aged for a minimum of two years in charred oak barrels made from the trees of the nearby Limousin or Tronçais forests.
All Cognacs are blended which is what gives them their complex flavour. Prior to blending, all cognac is still referred to as eaux de vie. These blended flavours can vary greatly depending on the age and types of barrels used for aging.