Original sin is a fundamental concept of Christianity. It is an idea that distinguishes the religion from the other two Abrahamic faiths.
All three Abrahamic faiths believe that when Adam and Eve fell by disobeying God's order not to eat the fruit of the tree of the knowledge of good and evil, they rendered the Creation imperfect, introducing sin, pain and death into the world. For this they were punished by being expelled from the Garden of Eden. Christians hold that in addition, the guilt of their original sin is passed on to all humans: that from the moment of birth, humans are already distanced from God by being in a state of sin. (This is why a baby is baptized - the taint of original sin is washed away in a religious ceremony.)
The importance of original sin in Christianity is that Jesus is seen as the remedy for it: God sent "His only Son" into the world so that, by His death, the sin of Adam is purged, or redeemed. The point is made by Saint Paul in I Corinthians ch. 15, v. 8: "For since by man [Adam] came death, by man [Jesus] came also the resurrection of the dead. For as in Adam all die, even so in Christ shall all be made alive."
Although Christian theologians agree that original sin is a fundamental feature of human nature, they give different accounts of what it consists in, i.e., the ways in which it affects, and ‘spoils’, human nature. Some (e.g., the early Augustine (354-430) and theologians in the Greek Orthodox Church) view original sin as (no more than) a (relatively weak) inclination to commit sinful acts; others (e.g., the later Augustine and many of the Protestant reformers of the 16th century (such as Martin Luther (1483-1546) and John Calvin (1509-1564)) take a darker and more disturbing view, holding that through original sin human nature has become thoroughly wicked and depraved.
Ancestral sin is an alternative expression for original sin.