Sin
From Neoredemptive
The best definition we have found for "sin" comes (naturally) from scripture:
Whatever does not come from faith is sin. (Romans 14:23)
Any deed, any failure to act, any thought, any word, any inclination of the heart, anything at all that touches on our lives individually, as church congregations, as the church at large, as communities, as civilizations, or as humanity taken together, undertaken with any motive other than complete and wholehearted faith in God our creator, provider and master, and His Christ our redeemer, is by its nature sin.
Most disturbing about such a definition of sin is that it refuses to be content to think of sin as something "out there", or merely something that we do. Sin permeates human life, human thought, human behavior, human will, the whole of the human experience; our "sin problem" is not a need to change our behaviors, but rather a need to completely renovate the human condition from the inside out.
It is rightly terrifying to realize that there is no neutral ground in the universe. From our most deeply kept secret thoughts to the grandest of galaxies, there is nothing that simply "is what it is"; everything is, at every instant, either in rebellion against or submission to its maker. This is why "the righteous shall live by faith" (Heb 10:38 &c) - because everything not ruled first and foremost by that faith can only be sin and death, destruction and disarray, decay and emptiness.
Original Sin and Total Depravity
Original Sin, in a nutshell, is the doctrine that Adam and Eve represented us well and accurately when they sinned against God in the garden.
Total Depravity simply asserts that the destructive power of sin -- of internally displacing God from His throne and seeking to place anything else atop it -- is devastating to the entirety of the human condition, and that there is nothing in our nature, our abilities, or our faculties which has not been marred and distorted by it. While we still bear the Imago Dei, we now do so imperfectly and unreliably. We can love beauty, but we cannot always discern it from wretchedness. We can love justice, but we do not always esteem it above power. We can love one another, but we cannot escape doing so in selfish ways for selfish reasons. We can reason, but we can neither find the basis for our rationality nor consistently bind ourselves to its consequences. While Christ has broken the power of sin and death and redeemed us from its curse, we still must live in the "time between the times", when the eschatological future is inbreaking, but still seen only "in a mirror, dimly".
Other References
If you're interested in a secular, anthropocentric, watered-down "World Religions 101" perspective on sin, take a look at Wikipedia: wikipedia:Sin.

