Eris was, as her name clearly indicates, the goddess of discord (eris). According to Hesiod she was the daughter of Nyx. She is famous for her part in the marriage of Peleus and Thetis: at the gathering she threw a golden apple inscribed 'for the fairest' among those present.
Zeus suggested that the three goddesses who claimed the title, Hera, Athena and Aphrodite, should ask Paris to decide who was the rightful winner. Aphrodite was selected, and this was one of the main causes of the Trojan War. The apple was the proverbial 'apple of discord'. The Roman equivalent goddess was named, predictably, Discordia.