Being married to a man who spent a year working on a Cognac vineyard in France, I have received the Cognac- is- NOT - JUST -brandy story from him, from his French family, from all of the neighbors who also produce Cognac, from the tours of Martell and Remy Martin, the Cognac musuem in Cognac ad nauseum. I am compelled to share the love
All Cognac is brandy, but not all brandy is Cognac. Brandy is simply distilled wine. Cognac is grape brandy which is produced in the Cognac region, from specific grapes (ugni blanc), follows a strict distillation and storage protocol and claims the title of the finest of brandies.
Blah blah blah to me, who doesn't like the stuff neat (gasp), but we always some on hand and I DO like it in recipes- it certainly has a distinctive taste. If you are anal about such things the distinction could be important, as Grappa, Marc, Armagnac, Calvados, Metaxa are all brandies but of very different flavors/qualities. Armagnac would be the closest to Cognac.
There are also non-alcoholic subs (orange juice?), but Ralph's link should be all that you need.
More than you wanted to know?
If you are a dreamer, come in. If you are a dreamer, a wisher, a liar, a hoper, a prayer, a magic-bean-buyer. If you're a pretender, come sit by my fire, for we have some flax-golden tales to spin. Come in! Come in! Shel Silverstein, Where the Sidewalk Ends.