Treasure Fleets vs. Treasure Ships

A "Treasure Galleon" is a unique Spanish ship type that has extra cargo room. They'll have the regular types of loot (you can usually deduct what they're carrying by what port they sailed from -- whatever that port/colony normally sells, they'll be carrying it), just more than usual.

A Treasure Ship means it's one of the Spanish (treasure) galleons that carries the massive amounts of gold and silver that the Spanish mined/looted from the West Indies back to Spain. They carry a great deal of money as well as treasure, usually several thousand at least.

Needless to say, capturing one will make you very rich, your crew very happy, and Spain very annoyed with you.