Sin  
Sin is a term used mainly in a religious context to describe an act that violates amoral rule, or the state of having committed such a violation. Commonly, the moral code of conduct is decreed by God .  Sin is a major foundational concept in Judaism, Christianity, Islam and other religions. It is certainly one of the most important themes in the Old and New Testaments.

Most of these religions do not define the word sin in any way that might be clear to those not of their faith.  Sin can best be described as some sort of mud or filth that adheres to one's soul when one violates one of the laws, rules and regulations of that particular faith. This coating of mud is believed to keep one from entering heaven.
Many Eastern religions view sin very differently. Sin is viewed as an error caused by inadequate knowledge. Many teach the concept of Karma. This is the total effect of the good deeds and sinful behavior which each person accumulates during their lifetime. These religions generally teach that, after death, one's soul enters a new body in order to live a new lifetime. The nature of a person's next reincarnation will be determined by their accrued Karma at the time of their death.
Many Newagers, Wiccans and other Neopagans have also incorporated belief in Karma into their religions. However, most believe that the impact of sin and of good deeds only extend to a person's present lifetime. Many Neopagans tend to define sin in terms of actual harm done by one believer to themselves or to other people. Many believe in the Threefold Law by which the universe is organized to return any harm that a believer had done to others, increased three times in severity. This heavily motivates them to not harm, injure, dominate, manipulate and control others.

 
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. 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."
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."

In other words,

James, who was perhaps the wisest of all New Testament writers, speaks of "The Perfect Law of Liberty", but he does not define the term.  It is Paul who clarifies the Law of  Liberty when he says, "to the pure all things are pure."

Again, it is not any act that we might perform that makes us sinner,  but rather sin is the state of our mind.
For a full theological explanation see: Adam & Eve