The Original Sin refers generally to the Christian belief the universal nature of sin. Original sin is considered to be the result of the story of Adam and Eve in The Bible. In that story (Genesis 3, they broke God's one and only command (not to eat the fruit from the tree of the knowledge of good and evil) when the snake persuaded Eve that eating that fruit would make her and Adam like God. Once they broke God's solitary command, they were removed from the Garden of Eden.
Then Genesis tells the story of how their oldest son Cain killed his brother Abel. Based mostly on this and many other stories and passages in the Bible and Saint Paul's statement in Romans 5:12 (Sin entered the world because one man, Adam, sinned. And death came because of sin. Everyone sinned, so death came to all people.--NIV) Christians traditionally believe that no human is without sin.
More specifically the term "Original Sin" refers to the sin a person has not because of any specific act they have done personally but because of they are born into the human race. This was first explained fully by Saint Augustine in his writings against the Pelagians. The Eastern Orthodox Churches do not believe in this specific doctrine of Original Sin.