The Westminster Catechism defines sin as "any want of conformity to or transgression of the law of God." We notice here that sin is defined both in negative and positive terms. The negative aspect is indicated by the words "want of conformity." It points to a lack or failure in moral performance. In popular terms it is called a sin of omission. A sin of omission occurs when we fail to do what God commands us to do.
The positive aspect of the catechetical definition of sin refers to overt, actual stepping over the boundaries of God's law. It is a sin of commission.
Sometimes God expresses His laws in negative terms (do not . . .) and sometimes in positive terms (do . . .). The Ten Commandments contain both forms (do not steal; honor your father and mother).
Both sins of omission and sins of commission are real sins. They incur real guilt. When we do what God forbids, we are guilty of a sin of commission; when we fail to do what God commands, we are guilty of a sin of omission. In both cases the law of God is violated.
Prayerfully examine your life for sins of omission or commission.