What is Sin?

"So for one who knows the right thing to do and does not do it, it is a sin" (James 4:17)
I think sometimes we make things more complicated than they really are. This verse says it all. It reminds us that if someone does not know what is right, they have committed no sin. This of course is different than trying to justify ones actions (which is usually a sign that deep down, they know it's wrong). This same verse also holds us accountable to some things that may not be considered sinful when following the exact letter of the law.
This is a wonderfully simple definition of sin - basically, follow your conscience.
There are times that we do not do the right thing because our conscience is not fully or properly formed (and we should strive to correct this) but it seems to me we usually know the right thing - we just don't do it.