Yom Kippur is considered by Jews to be the holiest and most solemn day of the year. Its central theme is atonement and reconciliation. Eating, drinking, bathing, anointing with oil, and marital relations are prohibited. Fasting begins at sundown, and ends after nightfall the following day. Yom Kippur services begin with the prayer known as "Kol Nidrei", which must be recited before sunset. (Kol Nidrei, Aramaic for "all vows," is a public annulment of religious vows made by Jews during the preceding year. It only concerns unfilled vows made between a person and G-d, and does not cancel or nullify any vows made between people.)
A Tallit (four-cornered prayer shawl) is donned for evening prayers; the only evening service of the year in which this is done. The Ne'ilah service is a special service held only on the day of Yom Kippur, and deals with the closing of the holiday. Yom Kippur comes to an end with the blowing of the shofar, which marks the conclusion of the fast. It is always observed as a one-day holiday, both inside and outside the boundaries of the land of Israel.
(Adapted from Wikipedia)