The Elements of Harmony represent the strongest magic known to the ponies of Equestria.

Each of these elements is represented by one of the six main ponies in the series.
The elements included are:

Loyalty - Represented by Rainbow Dash,
Honesty - Represented by Apple Jack,
Generosity - Represented by Rarity,
Kindness - Represented by Fluttershy,
Laughter - Represented by Pinkie Pie,
Magic - Represented by Twilight Sparkle,

The Elements were used against Nightmare Moon when Princess Celestia imprisoned her in the moon. They were then used against her again by the six main characters to stop her from bringing on eternal night.

The last Element "Magic" must be activated by friendship. The Elements Of Harmony are restored in the Tree Of Harmony, deep within the Everfree forest.