"Let the reader, ... where he finds me
erring, call me to his side. So we may keep
to the path, in love, as we fare toward Him 'whose face is ever to be sought.'"
- St. Augustine, De Trinitate 
I. Roman Catholic doctrine
Original sin, according to the traditional and still official Roman Catholic doctrine, is "the hereditary sin incurred at conception by every human being as a result of the original sinful choice of the first man, Adam." 
Original sin is considered to be a condition that affects man prior to his own personal sin, but it is not to be identified with concupiscence , which makes man prone to personal sin. It is not "actual" sin, but an "habitual" state of sin. 
There is no official teaching of the Catholic Church as to the nature of original sin: "whereas we know that there is an original sin, we do not know for certain what it is." 
Medieval theology described the nature of original sin as a lack of the grace and privileges enjoyed by Adam and Eve before "the fall."  Schmaus describes original sin as an incapacity for dialogue with God.  Rahner says the essence of original sin is that sanctifying grace is not available to man in making moral choices -- unavailable not through a positive action of God in withholding a free gift but through a negative condition of man that prevents infusion of the gift. 
Original sin is "washed away" by baptism ; but the sin is still passed on from the baptized to their offspring by "generation," that is, through the propagation of the species. To use a medical analogy, the baptized are no longer afflicted with the malady but remain carriers.
The doctrine of original sin is intimately tied up with other theological matters including personal sin, salvation, grace, baptism (especially infant baptism), and even the origin of the human species.
The traditional doctrine is based on three assumptions: 
1) The early chapters of Genesis contain a factual account of the origins of the universe and of humankind.
2) Genesis 3 narrates a literally true story of how the first man and woman sinned against God.
3) Adam and Eve were punished in a way that would affect all their descendants.