Dragonkin are all creatures descended from and including dragons. Dragonkin sometimes refers specifically to a type of creature closely related to a dragon, but usually lacking wings or full dragon abilities, powers, and spells. Nearly all dragonkin belong to one of the main or lesser dragonflights.
As a more specific term
At times dragonkin may refer exclusively to non-dragon dragonkin.
The term dragonkin has been used to refer to a category that seemingly includes both dragonspawn and drakonids. Some dragonspawn are known as "dragonkin", like the Elder Dragonkin and the Twilight Dragonkin. They are called such as a whole in  The Essence of Enmity. Maxnar the Ashmaw, a drakonid, is described as "one of the dragonkin, half-man, half-dragon." The dragonkin originate from when dragons empower mortal servants, changing them into something halfway. This process does not have to be willing. They became dragonspawn and drakonid thanks to the dragons they served, via intentional alteration or via unintended side effect of hanging out with dragons.
Tyri at the Celestial Ridge also calls the nether dragons there "dragonkin".
Characteristics in World of Warcraft
- Reptilian and almost always have a tail.
- Scaly skin that is usually skinnable.
- Dragon scales often have magical properties and are highly valuable to craftsmen.
- Have sharp claws.
- Semi-intelligent to very intelligent.
- Some can fly and usually have bat-like wings in that case.
- Some have a breath-weapon.
- Some have special abilities or powers and can cast spells.
- Most are susceptible to the Druid's [Hibernate] ability.
- Whelp - The infant stage of the dragon race
- Drake - The adolescent stage of the dragon race
- Dragon - The adult stage of the dragon race
Types of Undead Dragonkins
Dragonkin that don't have any relation with dragonflights, and as such maybe just classified as Dragonkin for gameplay purposes.