What is Sin?

Have you ever gone to a restaurant and when you first sit down, the desserts on the menu looked sooo good, but, by the time you finished your meal you couldn't stand the thought of dessert? If you find a good book to read or hobby to do, you'll find you don't have time to sit and mindlessly watch TV for hours. I was thinking about this idea yesterday, that starting good habits often crowd out the bad ones. The scripture came to me, James 4:17 "To him that knoweth to do good, and doeth it not, to him it is sin." So, I was thinking, is it the wrong things that we are doing that are sin, or is it the failure to do good? It seems that when people are not being obedient to doing what is good or right, they just naturally seem to fall into sinful habits. On the other hand, if we are out feeding the hungry, caring for the sick, leading the lost to Christ, reading the Bible, praying, taking care of our families, etc. how would we have time to sit around gossiping, overeating, partying, watching things we shouldn't, etc. The scripture plainly says that if we are not doing what we know to be good, we are sinning. Maybe, we should quit focusing on the sins the enemy is condemning us with, start doing good, and soon we won't have the time or desire for the things that hinder us. So what do you think, what is sin? Is it the bad things that we do, or the good that we don't do?