This class (or in Final Fantasy Tactics, a job class) of warriors, use the abilities of the dragons to attack their enemies from the air by landing on them, lance first. This leaves the dragon knight temporarly invincible, but some enemies are quick enough to avoid their attacks. Sometimes it takes Dragon Knight several turns for them to land, leaving the rest of the party to deal with the enemy. Sometimes they land after the enemy(s) is/are defeated. In the Final Fantasy series (which I'd like to remind you is a copyright of Squaresoft. I don't want to be slapped with a lawsuit or anything), they're known as Dragon Knights, Dragoons, or Lancers.
The picture on the right is two Dragon Knights from Final Fantasy Tactics