Brandy and cognac are both distilled spirits made from fermented fruit juice, typically grapes, but there are key differences between the two:

Brandy is a broad category of distilled spirits made from fermented fruit juice.  While grapes are most commonly used, brandy can also be made from other fruits such as apples (apple brandy), pears (pear brandy), and cherries (cherry brandy).

Brandy can be produced anywhere in the world. Notable brandy-producing regions include Spain (where brandy de Jerez is made), the United States (particularly California), South America, and various European countries.  There are many varieties of brandy, each with unique characteristics depending on the fruit used and the production methods. Examples include Armagnac (from the Armagnac region in France), Calvados (apple brandy from Normandy, France), and Pisco (from Peru and Chile).  Brandy is typically aged in wooden casks, which impart flavors and colors to the spirit. The aging process and the type of wood used can significantly influence the final product.

Cognac is a specific type of brandy that must be produced in the Cognac region of France, following strict regulations.  Cognac must come from the Cognac region, which is divided into six sub-regions or crus: Grande Champagne, Petite Champagne, Borderies, Fins Bois, Bon Bois, and Bois Ordinaires.  Cognac must be made primarily from specific grape varieties, with Ugni Blanc being the most common. Folle Blanche and Colombard are also used to a lesser extent.  Cognac must be distilled twice in copper pot stills.  Cognac must be aged in French oak barrels for a minimum of two years, though most are aged longer.

The age classifications include VS (Very Special, at least 2 years), VSOP (Very Superior Old Pale, at least 4 years), and XO (Extra Old, at least 10 years as of 2018).  Cognac is known for its rich, complex flavors that can include notes of fruit, vanilla, spices, and oak, depending on the aging process and the specific cru where the grapes were grown.

Brandy can be made anywhere, while cognac must come from the Cognac region of France.  Cognac has strict regulations regarding grape varieties, distillation methods, and aging requirements. Brandy production is less regulated and more varied.  Cognac is generally considered a higher-end product with more refined production techniques and aging processes, contributing to its reputation for quality and complexity.

In summary, all cognac is brandy, but not all brandy is cognac. The distinction lies in the specific production methods, geographic origin, and regulatory standards that cognac must adhere to.