Cognac and Armagnac are the two specific types of Brandy. The main difference between these two is also known as the golden rule of real estate: Location, location, location. Only a Wine Brandy produced in Cognac region can be called Cognac, while Armagnac must be from the Armagnac region in Gascony, southwest France. There are, of course, a few other things specific to their production that distinguish Cognac and Armagnac:

Nearly all Cognac is made of the grape variety called Ugni Blanc (Trebbiano). The same type of grapes is also used for Armagnac, but there are three other varieties that are equally common. All of them are white grapes.

And last, when it comes to taste, Cognac is more subtle and gentle, while Armagnac is considered to be more complex and robust. It's also higher in alcohol; Cognac must be at least 40% ABV and Armagnac is typically between 46 to 48% ABV.

 
Aging meaning
 

VS (Very Special)—no younger than two years old. Most VS Cognacs are designed to be drunk long (with, say, ginger ale or tonic and a dash of bitters) or in a cocktail or when cooking with brandy only VS should be used.
 

VSOP (Very Superior Old Pale)—must be at least four years old. VSOPs can either drunk neat or be mixed for cocktails and long drinks 
 

XO (Extra Old), as well as bottles labelled ‘Napoléon’, ‘Extra’ and ‘Hors d’âge’, it’s six to ten years. They tend to be best enjoyed neat or adding ice to Cognac to enhances the flavors and aromas.