Starlight is a unicorn pony, and the daughter of Princess Celestia. Before anyone says anything, her evilness has nothing to do with her mother.
Starlight was born around the same time as Sword Dancer, and was originally a good kid... it wasn't until she reached school that she started getting evil.
The other students treated her as a queen bee, and this all went to Star's head. She thought, "If I can have the school... why not Equestria?". She told her aunts and mother about her idea, and they didn't take her seriously... this enraged her, and a plan was put into motion.
She gained a tyrannical grip on Equestria, and imprisoned her family into their sources of power. Her position was gained because everypony underestimated her and never used their full power, and by the time she had it nopony could do anything to stop her.
Eventually, Starlight may see the error of her ways... but it'll take a stallion to do it.
Star is very vain and snobby. She's full of herself and determined, and slightly insane and evil. Star sees nopony as a friend, though she may cast a spell to make you think that she's your friend.