Honestly, Buddhism puzzles the hell out of most people in the West. (Actually, it puzzles the hell out of people everywhere, but we'll stick with the West for today.) When Shakyamuni Buddha (which is to say this guy Siddhartha that people talk so much about) was asked about any of the big questions religion is supposed to answer, his reply was "don't bother your pretty little head about such things, sugar."
Well, strictly, it was a little more formal than that, but that was the basic answer. There was an old ascetic, Vacchagotta, who came to the Buddha one day and started asking the big questions about the world, about life after death, whether the Universe was limited or infinite, and so on. Vacchagotta was from a different group, and he was trying to get Shakyamuni to engage in debate, and Shakyamuni wouldn't do it. Vacchagotta would ask, and Buddha would just stay silent.
Finally, Vacchagotta asked Buddha "why won't you answer?" and that question Shakamuni would answer. He told Vacchagotta that if he were to answer any of those questions one way or the other, then he would be siding with one side or the other in various big theological debates. From these debates, he said, arises disputes and arguments and contention and suffering. So he wasn't going to answer at all.
Buddha was pretty cagey that way: when people tried to nail him down on big philosophical questions, he always came around to ask "is this helpful in ending suffering?" So there's really no concept of sin in Buddhism, because there's no concept of pure good and evil. Instead of Commandments, we've got the Precepts, which are just guidelines: do this, and you will tend to reduce suffering.