Three styles
Potstill, vintage, and blended. Cognac is a place, not a method.
The classes below are the split published by the SA Brandy Foundation. Their page explains distillation. This page is only the difference that matters when you choose a bottle.
Potstill
100% potstill spirit. At least 3 years in oak of at most 340 litres. At least 38% ABV. Drink it neat, with ice, or with water.
Vintage
30–80% potstill brandy plus matured wine spirit. At least 8 years. At least 38% ABV. Drink it the same way as potstill.
Blended
At least 30% potstill brandy, that portion at least 3 years old, plus unmatured wine spirit. At least 43% ABV. It is made to mix with cola, ginger ale, or fruit juice.
Cognac
Cognac is brandy from the Cognac region. A Cape potstill can be made the same way and cannot be called cognac.