The opening prayer of Yom Kippur is the Kol Nidrei (or Kol Nidre) "annulment of vows" recited at sundown of Yom Kippur eve.
The Kol Nidrei service consists of the opening of the Ark and taking out the Torah scrolls, reciting the Kol Nidrei and returning the Torah scrolls to the Ark.
Kol Nidrei, the prayer which ushers in the holy day of Yom Kippur, is perhaps the most famous one in our liturgy. Ironically, it is not really a prayer at all, but rather a statement. A statement that deals with promises, vows and other sorts of verbal commitments commonly made in the course of the year. The Torah places strict demands on keeping one’s word, and not fulfilling a vow is considered a serious misdeed.
Kol Nidrei, which means "all vows", nullifies the binding nature of such promises in advance. One declares all future vows and promises invalid, by declaring that all vows are "absolved, remitted, cancelled, declared null and void, not in force of in effect."
On Yom Kippur when the essence of the soul is fully revealed, we express our real attitude towards the imperfections which might slip into our behavior, in the coming year. They are thus denied and declared insignificant.
The evening service which follows Kol Nidrei consists of the Half-Kaddish, the Shema, the Amidah, the Al Chet confession of sins, and special additional prayers (piyyutim) which are said only on the night of Yom Kippur.
Many have the custom to recite the entire Book of Psalms after the evening service.