Sin nowadays refers almost exclusively to evil or--non-religiously--merely naughty
actions, such as murder, rape, or eating chocolate instead of dieting.
This semantic problem makes explaining other kinds of sin, such as Original Sin, because it's become a hot-button word puts people on the defensive, as opposed to being the technical term that Christianity uses it as.
At its most abstract, sin is separation from God. (This is why, by definition, God is sinless.) Not actually a physical separation--although this is also an effect of it--but a sort of conceptual separation. That said, there are two different ways to be separated from God.
First, by direct action. This would be disregarding any of the Ten Commandments or the Golden Rule or the like, actions based in hate or in twisted forms of love. You probably already know about this.
Secondly, by inheritance. If you come from something that's been separated from God--and in Christian theology everyone has--then you also have this separateness condition. It doesn't mean you've done anything wrong yourself, although it does give you a predisposition to do so. This second kind of sin is why even perfect people need salvation, and it's called original sin.
This sin-system is actually something like how children of alcoholics are more likely to become alcoholics themselves. Of course, if they don't drink at all, they won't become alcoholics, but they may still have the tendency.
Yes, this is a "Christian-biased" node. Node what you know, you know. Let me know if I've botched the theology of it.