The High Holiday in The Jewish Tradition is a time to make your peace with God. It began at sundown last night and continues through sunset today. I grew up observing this holiday very strictly, which involves fasting, self-reflection and spending alot of time sitting in Shul. I am not observant, as the childhood torture of the endless sitting convinced me that it was a man made tradition and that God wouldn’t make children sit for so long. But I do feel the cultural and historical obligation to mark this holiday. Judaism is very specific about what sins can be forgiven, and what can’t.
Yom Kippur is not a time to repent sins against other people, our belief is that you have to actively reconcile problems with other humans.
What then, are sins against God? This is a fascinating topic, and something to ponder.
http://www.wisegeek.com/what-is-yom-kippur.htm