From what we understand, the main differences between frangipane and marzipan are what gets mixed into the almond paste and how they’re used.
Frangipane is similar to a pastry cream and is used in much the same way. The almond paste base is enriched with sugar, butter, and very often, eggs. This makes a soft, spreadable cream that’s used almost exclusively as a filling for pastries like croissants, fruit tarts, and our new favorite bostock.
Marzipan is an almond candy. Powdered sugar and corn syrup (or glucose) get worked into the almond paste to form a pliable, clay-like substance similar to fondant. This is used to sculpt the miniature fruits and detailed animals that decorate the tops of cakes or get sold as stand-alone candies. Dyes and edible paints can be applied to the candies to make them look even more life-like!
You can buy both frangipane and marzipan, but they're also easy to make at home. Martha Stewart has tutorials for both: