Titans are a mysterious race that hunts only humans for food and pleasure.
Titans are classified by their height.
Titans are enormous humanoid creatures who have no secondary sexual characteristics such as genitalia or breasts. Their bodies are at high temperature, and they have a high healing factor. A cannon shot to the head does not kill them. Their weaknesses is their nape. Humans have to cut deep into the nape of the Titan in order to kill them. Upon killing a Titan, their body decomposes and give out steam that is known as a Titan Vapor. Despite being large, their bodies are actually light. They do not have any organs. Their stomach cavity holds the dismembered corpses of their prey. When the Titans get full, they regurgitate their food and resume eating and killing humans.
There are humans who can transform into Titans.
Little is known how humans can transform into Titan. Speculation points to the mysterious drug that Dr. Jaeger injected into Eren after the Shiganshina District incident.