Calvados is the reference most English speakers have for apple brandy, and it is the right starting point. Lambig belongs to the same family: cider, distilled, then aged in oak. The difference is where it comes from. Calvados is Normandy’s, lambig is Brittany’s, and each has its own appellation, so neither name can go on the other’s bottle. In Brittany, lambig is simply the local equivalent: the brandy you are offered at the end of a Breton meal.
The two regions even share a custom. The trou normand, a glass of calvados halfway through a long meal, has a Breton twin, the trou breton, done with lambig. Pommeau follows the same pattern: Normandy makes one too, under a separate appellation, and if you have had pineau des Charentes, pommeau is the apple version of the idea.