Yom Kippur is probably the most important holiday of the Jewish year. The name "Yom Kippur" means "Day of Atonement." It is a day set aside to "afflict the soul, to atone for the sins of the past year.” This day is, essentially, your last appeal, your last chance to change the judgment, to demonstrate your repentance and make amends.
On Yom Kippur people atone for sins between humans and God, not for sins against another person. To atone for sins against another person, you must first seek reconciliation with that person, righting the wrongs you committed against him or her if possible. That must all be done before Yom Kippur.
Yom Kippur is a complete Sabbath; no work can be performed on that day. It is a complete, twenty-five hour fast day; beginning before sunset on the evening Yom Kippur begins, ending at nightfall on the day of Yom Kippur. The Talmud also specifies additional traditional restrictions: washing and bathing, anointing one's body (with cosmetics, deodorants, etc.), wearing leather and engaging in sexual relations are all prohibited on Yom Kippur. As always, any of these restrictions can be lifted where a threat to life or health is involved. Most of the holiday is spent in the synagogue, in prayer. Services end at nightfall, with the blowing of the tekiah gedolah, a long blast on the shofar.