Unicorns are one of the three main species of ponies that live in Equestria. Most of them live in Canterlot and the others around Equestria. All of them have a horn on their foreheads, some sharper and some longer than others. Their horns can be used to either charge after things or use it for other purposes. They are mostly associated with magic, each with their own. Each of the unicorns' horn glows during a spell is associated with their eye color. For example, Rarity's glow would be blue.
Unicorn magic, is limited to all unicorns. It depends on what their cutie mark is on how much magic they have. For example, Twilight Sparkle is said to be the most powerful of all Unicorns with her expansive variety of spells. Rarity has a gem-finding spell that uses to detect gems.
Some of this page is derived from the Friendship is Magic Wiki.The original article is at Unicorns.