Jews all over the world, even those who do not observe any other Jewish custom will (hopefully) refrain from work, fast and go to shul (synagogue) on Yom Kippur.
The holiday is mentioned in Vayikra Ch. 23:26 (et seq). It is a day set aside to "afflict the soul," to atone for the sins of the past year. In The Story of Rosh Hashana, we mention the "Books" in which Hashem inscribes all of our names. On Yom Kippur, the judgment entered in these "books" is sealed. This day is, essentially, your last appeal, your last chance to change the judgment, to demonstrate your repentance and make amends.
Learn all about the Yom Kippur - Story - Coloring Pages - Cards - Fun & Games http://www.torahtots.com/holidays/yomkipur/yomk.htm