Yom Kippur is a Jewish holiday designated by the Torah as the 10th day of Tishrei (October 7-8 in 2011).  It translates to the "Day of Atonement," which actually very effectively explains the tradition and intentions.  It's a day of cleansing of the body and  of spiritual rejuvenation; as the name suggests, one must also “atone” for one's sins in order to follow through with a complete renewal of spiritual healthiness and focus on God.   In doing so, one must deny the body its habitual luxuries.

Needless to say, Yom Kippur is considered the most important Jewish holiday and corresponds with the highest synagogue attendance rate of the year.  

Here’s how it started.  Moses came down from talking with God at Mount Sinai with his fresh imprint of the Ten Commandments, but many of his follows abandoned the law of God and were captured practicing something strictly prohibited—worshipping a golden calf.  God was angry and horrified with this behavior; however, after Moses' entreaties, God decided to forgive them. 

But only after they repented for their sins.  They needed to expiate by fasting and praying for forgiveness from God.  The punishment for failure to participate was not light.  God dictated to Moses "Those who do not deny themselves on that day must be cut off from their people" (Leviticus 23:29).  So the people repented; hence, the birth of the most widely practiced Jewish holiday.

But these requirements mandated by God are more specific.  In order to focus on one's spiritual needs rather than feed one's dependence on earthly and bodily attainments, practitioners of Judaism must follow a lot of rules.  Jews are required to fast, pray, refrain from sex and eschew Chanel and fancy lotions.  In addition, there are many liturgical practices to help with this process.  These include the annulment and dissolution of previous vows made to God and confession.