The part of the dragon that distinguishes itself from all other creatures. The dragon wing is a huge, leathery appendage that is found on several different species of dragon. The wing is usually larger than the dragon's body to accommodate the incredible force needed to lift and maintain flight.

Bones
The wing is essentially another arm and hand. If you look at the skeleton of a dragon front arm and claws, you will notice that the wing is just a very stretched out version. The two thick "arm bones" (humerus and wing radius) runs from the body of the dragon attaching itself with cartilage and muscle to an "elbow" of the wing. There are usually 4 or 5 elongated "fingers" on a wing each ending in a claw. One short "thumb" claw is at the "wrist". The "fingers" then attach themselves to the "wrist joints" to form the complete wing.

How does a dragon fly

The actual force of flight is the continuous down sweep of powerful wings scooping, and thrusting the body upwards. An immature dragon does not have the strength needed for a vertical jump from flat ground to take off. They will usually stay near higher cliffs to use the heat updrafts to keep aloft. It takes a few years of flying to build the extremely strong flight muscles needed to lift the dragon body from the ground. The wings make actually look small for the mass of the dragon, but all dragons have an innate magic ability to help them fly. This magic helps keep dragons from straining themselves during long flights. 

Weapons 

Dragons can also use their wings as an attack weapon if need be, but this is a last resort. 
The wing membranes are easily damaged compared to the rest of a dragon body.

If a dragon wing membrane is slashed, it takes a very long time to heal, and there is a chance the dragon might never fly again if it does not heal correctly. If attacked on the ground, the dragon will usually run then jump in the air and fly off avoiding the attacker. But if the dragon is trapped on the ground it will tuck its wings tightly against its body and use its breath weapon and claws to protect itself. If the dragon has to use its wings to attack it will scoop down and slash with its razor sharp wing claws to immobilize the attacker.