Yom Kippur is the Day of Atonement. Jews go before G-d and ask for forgiveness for the sins committed before Him over the preceding year. The mood of solemn contemplation and repentence is heightened by the rules of physical abstinence which require a full fast from all food and drink from dusk the previous day (known as Kol Nidre) until the end of the Yom Kippur service over twenty-five hours later. Whole books could be written about the symbolisms and traditions behind Yom kippur, but sadly we have only one Web page.
As well as fasting, Jews are not allowed to wear leather to shul on Yom Kippur, and should wear no jewellery or make-up. We are also forbidden sexual congress on this holy day. Normally, the kippah will be white, symbolising cleanliness.
One noticeable part of the service on Yom Kippur, which is split into main sections, is when the rabbi leading the service prostrates himself before the ark and asks G-d to forgive his sins. Two members of the congregation then help the rabbi to stand. This is the only time in the year in which it is permissible to kneel or to prostrate onesself on the ground. Absolute prostration (falling upon the face) is forbidden absolutely until the return to temple times.
Many years ago a scapegoat was used to rid a people of their sins. It would, in effect, have all of their sins placed upon it, and then it was cast into the desert or thrown off a cliff to be killed.
Fasting on Yom Kippur is not obligatory for the young (pre-Bar mitzvah age) or for the old or the sick. Special prayers can be recited instead of the fast for these people.