The best known and most common Yom Kippur prayer is called Kol Nidre ("all vows" in English). Kol Nidre is the prayer recited just before sunset, nine days after Rosh Hashanah.
While other prayers are made during Yom Kippur, Kol Nidre is considered the central prayer marking the day of atonement. It's an unusual prayer in that the words uttered are neither praise nor entreaty but are essentially an incantation of legal code. The Kol Nidre is also in Aramaic, rather than Hebrew.
The exact history of the prayer is somewhat obscure, but most scholars believe its message of renouncing oaths was a response to conversions that Jews had perhaps been forced to undergo at the hands of Christians or Muslims. Because Jewish religion views verbal oaths and commitments as serious metaphysical gestures, many feel that the ritual prayer developed as a reminder preceding the day of atonement that one must begin Yom Kippur with a clear conscience, having renounced any oaths made prior; as for example, a conversion to another faith under duress.
The music to which the prayer is sung by the cantor is an ancient melody, somewhat sad and haunting. Legend holds that the tune itself was "missinai," or came with Moses down from Mount Sinai.