There are two sub-classes of Orthian Dragons: True Dragons and Lesser Draconic. The currently known True Dragons include the Red Dragon, White Dragon, Blue Dragon and Green Dragon. The currently known Lesser Draconics, which are related species that are not considered True Dragons for a variety of reasons, include the Dracolich, Drake, Wyvern, Lindwyrm, Dragon Turtle and Dragonling. Finally are the Dragonborn, who are the offspring of a True Dragon (or Dragonling) and a mortal.
Kobolds are also a distant relative, of draconic lineage but rather far removed by time.
There are also Pseudodragons, an umbrella term for reptilian creatures that resemble and share characteristics with actual dragons but clearly are not.
Language
Most Dragons speak a language called Draconic, which is ancient with no known origins or earliest date of record. Some scholars believe that Draconic may even predate Ancient Troll and be the oldest Orthian language in existence, considering that interactions between Dragons predates civilisation (however, Celestial, the language of the Gods, obviously predates even this.)