Yom Kippur is the holiest and most solemn day of the year for most Jews ([Karaite Judaism|Karaite Jews regard Passover as the holiest day of the year, as do Samaritans). Its central theme is atonement and reconciliation. This is accomplished through prayer and complete fasting - including abstinence from all food and drink (including water), unless fasting is prohibited for medical reasons (e.g., Jewish law does not permit fasting by nursing mothers, diabetics, people with anorexia nervosa, etc.). Bathing, wearing of perfume or cologne, wearing of leather shoes, and sexual relations are some of the other prohibitions on Yom Kippur - all them designed to ensure one's attention is completely and absolutely focused on the quest for atonement with God. The fast and other prohibitions commence on 10 Tishri at sunset - sunset being the beginning of the day in Jewish tradition.
A traditional Aramaic prayer called Kol Nidre ("All Vows") is traditionally recited just before sunset. Although often regarded as the start of the Yom Kippur evening service - to such a degree that Erev Yom Kippur ("Yom Kippur Evening") is often called "Kol Nidre" (also spelled "Kol Nidrei") - it is technically a separate tradition. This is especially so because, being recited before sunset, it is actually recited on 9 Tishri, which is the day before Yom Kippur; it is not recited on Yom Kippur itself (on 10 Tishri, which begins after the sun sets).
The words of Kol Nidre differ slightly between Ashkenazic and Sephardic traditions. In both, the supplicant prays to be released from all personal vows made to God during the year, so that any unfulfilled promises made to God will be annulled and, thus, forgiven. In Ashkenazi tradition, the reference is to the coming year; in Sephardic tradition, the reference is to the year just ended. Only vows between the supplicant and God are relevant. Vows made between the supplicant and other people remain perfectly valid, since they are unaffected by the prayer.
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.
Yom Kippur is considered, along with 15th of Av, as the Happiest days of the year.