Yom Kippur is an important Jewish holiday. Even those Jews who do not observe other Jewish customs will follow some of the Yom Kippur traditions. Just like on Rosh Hashanah, nobody works and there are services at the synagogue all day. The entire day is also spent fasting (no eating).
The Low Down
Yom Kippur means "Day of Atonement." This is your last chance to change God's judgment, make amends, forgive and forget. There are 10 days to make good with God, starting on Rosh Hashanah and ending on Yom Kippur. However, this is only for those sins against God, not man. Any amends you have to make with people, have to be done before Yom Kippur starts.
Rules of Engagement
The holiday starts at sunset and ends at nightfall the following day. In 2011, Yom Kippur begins on October 7th. Only kids under the age of nine and women in childbirth (from the time labor begins 'til three days after) are allowed to eat. Of course, if not eating could seriously damage someone's health or create a life-threatening situation, then any of the restrictions can be broken.
Most of the day is spent in the synagogue praying, so it doesn't really matter if you smell funny. People usually wear white on this day and some people even wear a kittle - a white robe that some people are buried in.