Princess Luna is a blue alicorn princess, the younger sister of Celestia and the co-ruler of Equestria. She was banished to the moon for 1000 years by her own sister when she was corrupted by jealousy, and turned into Nightmare Moon. Before being sent away, she helped defeat Discord, rule Equestria and save them from the crystal empire.
After returning, she continued her duties as co-ruler and raiser of the moon. In addition, she also visited ponies dreams (including Sweetie Belle's and Scootaloo's.)
Luna is usually seen wearing a chestplate and tiara (dark blue in season 1, black in seasons 1-4) and light blue shoes. Her cutie mark is a splodge (again, dark blue in season 1, black in seasons 1-4) with a moon crescent on top, simmilar to Nightmare moons.