Edom was one of the Iron Age states adjacent to the kingdom of Judah. It is already mentioned during the reign of pharaoh Merenptah (1213-1203); Edomite nomads were allowed to cross the Sinai and enter Egypt. It is better known, however, from the first centuries of the first millennium BCE, when it was sometimes subject, sometimes at peace, and sometimes at war with the Judaeans. The books of Samuel and Kings are important sources, and so are the Assyrian documents.
During the Babylonian Exile of the Jews, the Edomites conquered the southern part of Judah, perhaps expelled from their own country by the Nabataeans, who were, since the fourth century, certainly in control of the original homeland of the Edomites. The new country of the Edomites is usually called Idumea, and was forcibly converted to Judaism by the Hasmonaean leader John Hyrcanus (r.134-104).
More about Edom can be found on this new webpage.