The original (or "first") sin was commited by Adam and Eve (see Book of Genesis Chapter 3). Orthodoxy believes that, while everyone bears the consequences of the first sin, the foremost of which is physical death (in this world), only Adam and Eve are guilty of that sin. Roman Catholicism teaches that everyone bears not only the consequence, but also the guilt, of that sin. This difference between the two Churches in their understanding of the original sin was one of the doctrinal reasons that led the Catholic Church to devise their dogma of the 'Immaculate Conception' in the 19th century, a dogma that is completely rejected by the Orthodox Church.
Sources and further reading
- The Original Sin/Consequences of the Original Fall - by HG Bishop Kallistos Ware]
- Original Sin/St. Augustine & Original Sin - Teachings of Orthodoxy - by Fr. John Matusiak, OCA: , , 
- Original Sin, a short historical note - by Dr George Bebawi
- What are the differences between Orthodoxy and Roman Catholicism? - by Father Michael Azkoul
- CATHOLIC ENCYCLOPEDIA: Original Sin
- Original Sin - Wikipedia