According to the Talmud, the celebration of Yom Kippur can atone only for sins between man and God. To atone for sins against another person, you must speak with that person and seek a reconciliation. At the very least, you must apologize for your actions and if possible, right whatever wrong you committed against them. This must be accomplished during the Days of Awe, prior to the onset of Yom Kippur.
In addition to teshuvah, the High Holiday period is a time for increased generosity and tzedakah. One ancient custom, known as Kapparot, is observed on the morning prior to Yom Kippur. Not all Jews follow this custom. The actual practice has changed over the years, but the meaning behind it remains the same -- it's an act of penance for committed sins. To perform Kapparot today, you take a handkerchief or small cloth sack filled with money -- symbolizing compensation for your sins -- and swing it around your head (as if offering it up to God) while reciting a short prayer. Once you've performed the ritual, you give the money to charity.
Yom Kippur is said to be the "Shabbat of Shabbats" -- the Sabbath of Sabbaths. As such, there is to be no work done on this day. There are other restrictions, as well. Unless health dictates otherwise, Yom Kippur is observed by a 25-hour fast that begins prior to sunset on the evening before Yom Kippur and ends shortly after nightfall on the day of Yom Kippur. During this time, you are not allowed to consume any food or beverage, including water. According to the Talmud, the fast also includes abstention from:
- taking a bath or shower (washing of any kind, really -- although many Jews make allowances for hand-washing after visiting the restroom)
- anointing your body with perfumes, scented oils, lotion, deodorant or cosmetics
- wearing leather shoes (It's quite common to see men and women worshipping in canvas sneakers or slippers.)
- sexual activity
To prepare for the fast, a meal much like a traditional Sabbath meal is served. They say a blessing over the Challah, a special bread that also accompanies the celebration of Shabbat, and after the meal they light the candles with an accompanying blessing. The candle lighting and the Challah blessing are both traditional acts that "bring in" the Sabbath and other holy days.
As a sign of purity, it is often customary for men and women to wear white on Yom Kippur. For example, men may wear white kippot or yarmulkes (fitted skull-caps). Also to signify purity, the scrolls of the Torah may be cloaked in white, and the Ark (where the scrolls of the Torah are stored) is draped with a white curtain.