THAT'S A GOOD QUESTION

What Is Sin - The Big Question
We live in a culture where the concept of sin has become entangled in legalistic arguments over right and wrong. When many of us consider "What is sin?" we think of violations of the Ten Commandments. Even then, we tend to think of murder and adultery as "major" sins compared with lying, cursing, or idolatry. 

The truth is that sin, as defined in the original translations of the Bible, means "to miss the mark." The mark, in this case, is the standard of perfection established by God and evidenced by Jesus. Viewed in that light, it is clear that we are all sinners. 

The Apostle Paul says in Romans 3:23: "All have sinned and fall short of the glory of God." 

In light of this, it does no good to compare ourselves to others. We cannot escape our failure to be righteous in our own strength. This is by God's design, because only when we understand our weakness will we consider relying on the atoning sacrifice of Jesus.

What Is Sin - A Biblical Perspective
Sin is mentioned hundreds of times in the Bible, starting with the "original" sin when Adam and Eve ate of the tree of knowledge. Often it seems as if sin is simply the violation of any of God's laws, including the Ten Commandments. 
Paul, however, puts this in perspective in Romans 3:20, when he says, "Therefore no one will be declared righteous in His sight by observing the law; rather, through the law we become conscious of sin." 

God wanted us to recognize our sins. Even those who have not murdered or committed adultery will find themselves convicted of lying, or of worshipping false idols like wealth or power ahead of God. 

Tragically, sin in any amount will distance us from God. 

"Surely the arm of the LORD is not too short to save, nor His ear too dull to hear," says Isaiah 59:1-2. "But your iniquities have separated you from your God; your sins have hidden His face from you, so that He will not hear." 

We must resist the temptation to act as if we are righteous, especially by leaning on our good works.

"If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. If we claim we have not sinned, we make him out to be a liar and his word has no place in our lives" (1 John 1:8-10).

What Is Sin - A Call to Repentance
The good news in all of this is that, once we recognize ourselves as sinners, we need only to repent and embrace Jesus to be forgiven. Jesus can forgive us because he died and rose again three days later in victory over sin and death. 

The Apostle Paul refers to this process of recognizing sin and being responsible for it as "godly sorrow." 
"Godly sorrow brings repentance that leads to salvation and leaves no regret, but worldly sorrow brings death," Paul writes in 2 Corinthians 7:10-11. "See what this godly sorrow has produced in you: what earnestness, what eagerness to clear yourselves, what indignation, what alarm, what longing, what concern, what readiness to see justice done. At every point you have proved yourselves to be innocent in this matter."
http://www.allaboutgod.com/what-is-sin.htm