In 5e they're just humanoids. The default lore in the PHB is that they seem to be a fusion of dragon and humanoid performed by the gods to create a true race that can reproduce on their own, unlike the half dragon, which is basically a mule.

In the Forgotten Realms their origins are unclear, but they're said to be created by Io distinct from dragons, hence why they have a draconic connection while not being considered part of the "dragon" type.