The galleon has two features that distinguish it from other round ships: masts and prow. Like other round ships, it has a mainmast and foremast, but the rear mizzenmast is lateen-rigged. It sometimes has a fourth, even smaller rear mast. This lateen style allows superior sailing that could save days or weeks over long voyages in the open ocean. An instantly recognizable feature is a long beak jutting forward from the prow.
For authors, these features require explanation that may gain our audience little in understanding. This is especially true of the beak, which has no functional benefit aside from helping sailors tend to sails, which few readers will care about even if we understand the details of such things and wish to convey them. The beakhead is where sailors crapped, the waste falling directly into the sea, but this is true of all round ships, not just the galleon; oddly, it is the rear of round ships that has an area called “the poop.” This is also from where the expression “I gotta hit the head” originates, as the latrine is at the head of the ship and therefore downwind.
If we wish to explain to our audience what a galleon is and how it differs from other round ships like the frigate, the difference between square and lateen-rigging and the possible impact on maneuverability and speed is what to mention.
Since our world might not have guns, a new name might be needed for this small vessel that carries one or more large cannons (or whatever missile weapon we invent). They are cheap and easy to build and therefore more expendable. Used in coastal waters, they are no match for large ships, but a score of them can do horrific damage to a lone ship, which can’t sink them all fast enough. This boat is either powered by sails or oars.
A ship with sixty or more guns, which includes all first, second, and third-rate ships, the ship-of-the-line acquires its name from being designed for the line of battle when two fleets fight each other in two parallel lines. This allows for firing broadsides at each other without fear of hitting their own ships. Smaller ships are not considered strong enough to withstand such a barrage from these vessels, the largest ships afloat, and therefore aren’t part of the line, if present at all.
For an interesting cross-section of the interior of such a vessel, view this diagram: http://www.artofworldbuilding.com/warship.