Soul- a very common concept in many religions(which obviously borrowed from each other), is the spiritual and eternal part of a living being, commonly held to be separable in existence from the body; as distinct from the physical part. According to Hinduism and Buddhism it is indestructible and a part of God. When a being dies the soul escapes its physical confinement only to be recaptured and born again as another being. The cycle continues until the soul can achieve "Moksha" when the soul is permanently freed and rejoins the God.
But this is the belief of several of the religions.
What I believe is that soul does not exist. Its simply that all living beings are highly complex mechanisms, some like the humans even more complex. The animal kingdom consists of mechanisms that are self aware while the plant kingdom mostly does not. In fact these mechanisms are so difficult to understand, because we (the mechanism itself) is trying to understand it. Its like trying to measure the velocity of a bus while being inside the bus. That is why the early humans tried to imagine something external to the mechanism because that way it can be understood better. And thus came into being "Soul". Human beings obviously being at the highest tier of the evolutionary pyramid is the most complicated mechanism. As with any mechanism the kind of work that it can be put to do decides its value or utility the same with human beings. The kind of choices we make in life decides our value, utility and to some fools character. These choices are termed as "good" and "evil". Broadly defining(according to general morale) any kind of self-sacrifice can be called "good" while the remainder is "evil". We have to remember this classification has been done by the mechanism called human being itself. So the stories about going to hell or heaven upon doing evil or good respectively can be deemed to be all rubbish. For any mechanism to carry on it requires its raw materials. Same with us. We require the raw materials(basic commodities, services and luxury goods) and energy which are limited . Therefore the competition and the Darwin theory of "Survival of The Fittest". But there is another aspect to the whole story. That is we are self aware. The implications of this are vast. This makes us responsible for the development, continuance and evolution of our species which can not be achieved my doing what is best for oneself but doing what is best for the team. So while "self-sacrifice", "good", "evil" might all be rubbish, we need to do some of these rubbish for the sustenance of our kind. The best way to achieve sustenance of species as well as self satisfaction is through optimization.
Thus what we should do is optimize "good" and "evil" in our life and thus be satisfied as well as be a contributor to the overall mechanism of the universe.