Q:

What is the essence of sin?

There are bound to be lots of questions like "Is X or Y a sin?". However, what is sin at its root and where in the Bible do we get our understanding of it?

A:

Luther has famously argues that every time you break any other commandment, you always break the first commandment; in other words, behind every sin is the desire to worship a god other the God. Whether it is pride, in which we consider ourself to be God, or a sin of fear-of-man, in which we consider another person to be God, all sin is, at its core, idolatry.
When you see the essential problem as a worship issue, over against a breaking-of-commandments issue, I think several things are clarified.

Worship and idolatry addresses both the positive and negative sides. That is, when we do what is right, we are worshipping God (and conversely), and when we are not worshipping God, we are not doing what is right (and conversely).
Much of the Bible talks about idolatry, and seems a bit foreign and obsolete to modern ears. After all, we do not deal with Baal or Molech on a daily basis. However, when considering this as an example of our basic relationship with God, these passages become much more relavant. God is repeatedly enraged by his people worshipping idols and intermixing with other gods. Though we do not by and large bow down to physical idols made by hands, we do worship at the gods of money, power, etc.
The idolatry issues allows for a distinction between things that are good and created by God against things that ultimate for us. God gives us food, but we can be gluttons. God gives us sex but we find all ways to abuse it. Seeing sin as essentially idolatry allows us to have good things be God-centered, and not find abuse to be the only use we can comprehend.

A:

Simply put, sin (lowercase s) is the act of disobeying God.
A bit more complexly put, Sin (capital S) is kind of like a disorder that Adam and Eve got from disobeying God and that we all inherited.  This disorder (AKA our cursed flesh) contributes to our sinning, that is, committing sin, the act of disobeying God.

Requested Biblical references for original Sin:

Genesis 8:21
21 The LORD smelled the pleasing aroma and said in his heart: “Never
  again will I curse the ground because of humans, even though every
  inclination of the human heart is evil from childhood. And never again
  will I destroy all living creatures, as I have done.
Psalms 51:5
5 Surely I was sinful at birth,     sinful from the time my mother
  conceived me.
Romans 3:10-12
10 As it is written:
“There is no one righteous, not even one;   11 there is no one who
  understands;     there is no one who seeks God.  12 All have turned
  away,     they have together become worthless;  there is no one who
  does good,     not even one.”
Romans 5:12
12 Therefore, just as sin entered the world through one man, and death
  through sin, and in this way death came to all people, because all
  sinned

(All emphasis mine)
As to how this happened,  I don't know how better to describe it other than "we inherited it".

A:

To quote the Westminster Shorter Catechism (Q 14, "What is sin?"):

Sin is any want of conformity unto, or transgression of, the law of God.

And the references given (Leviticus 5:17, James 4:17, 1 John 3:4):

And if a soul sin, and commit any of these things which are forbidden to be done by the commandments of the LORD; though he wist it not, yet is he guilty, and shall bear his iniquity.
Therefore to him that knoweth to do good, and doeth it not, to him it is sin.
Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.

