The Elements of Harmony are a set of six magical objects which each represent a special quality associated with friendship. When wielded together, the Elements seem to possess almost unlimited power. They have been used by Princess_Celestia and her sister Princess_Luna to imprison the rampaging Discord in the form of a statue; by Princess Celestia to imprison Nightmare_Moon on the moon for a thousand years; by the Mane_Six to defeat Nightmare Moon and redeem Princess Luna; and finally, by the Mane Six again to defeat and re-petrify an escaped Discord.
Physically, the Elements originally took the form of six stone spheres, each marked with a rune or symbol. After these were shattered by Nightmare Moon, Twilight_Sparkle recognized that the true power of the Elements was housed in the spirits of her friends, and the Elements transformed into five necklaces and an ornate tiara.
The six qualities associated with the Elements (and the current bearer of each) are:
When the Elements are employed, it seems that the bearer of Magic is the one in primary control, although the presence and cooperation of all the bearers is necessary to activate the Elements. The power of the Elements manifests as a rainbow, but the exact effects depends on the enemy faced and the intent of the bearers.