The term billfish refers to a group of predatory fish characterised by prominent bills, or rostra, and by their large size; some are longer than 4 m (13 ft). Billfish include sailfish and marlin, which make up the family Istiophoridae, and swordfish, sole member of the family Xiphiidae. They are apex predators which feed on a wide variety of smaller fish, crustaceans, and cephalopods. These two families are sometimes classified as belonging to the order Istiophoriformes, a group with origins in the Late Cretaceous around 71 million years ago with the two families diverging from one and another in the Late Miocene around 15 million years ago. However, they are also classified as being closely related to the mackerels and tuna within the suborder Scombroidei of the order Perciformes. However, the 5th edition of the Fishes of the World does recognise the Istiophoriformes as a valid order, albeit including the Sphyraenidae, the barracudas.

Is it true that is a swordfish and a marlin the same?
no