There are three different categories of sin mentioned in the Bible. First, sins of commission are things we do outside of God will. Second, sins of attitude involve having ungodly thoughts and emotions, even if you are doing the right things. And finally, in our verse today we read about sins of omissions, which is not doing the things you know you should be doing.
So, whoever knows the right thing to do and fails to do it, for him it is sin.