The soul is the animating and vital principle in human beings, credited with the faculties of thought, action, and emotion and often conceived as an immaterial entity. It is this entity that gives human beings their spiritual nature. It is regarded as immortal, separable from the body at death, and susceptible to happiness or misery in a future state after the demise of the mortal form.
It is my personal belief that the soul is the very essence of a beings existence, but I don't believe that it exists in the form that we associate it with (a ghostly humanoid form). The more conceivable notion is that the soul exists as spiritual energy. This energy resides on a plane other than those in our 4-dimensional world.
This energy fuels the life force of the mortal form to which it is linked. Being that these souls are energy, it is viable to believe that they can be transfer energy not only to the human form, but also to other souls. This would then explain how one determines and finds a "soul mate." For example, a man's wife, if she is his true soul mate, can usually lift his spirits when he is depressed. Her soul transfers some of its energy to his through the link which they share. Since one's mood is directly associated to the amount of energy one expends, it can be reasoned that happy people have more energy, which is supplied by the soul.
This then leaves the phenomena of death. Since the laws of physics state that energy cannot be created or destroyed, how then does one die? The answer is very simple. Some force simply blocks the flow of spiritual energy to the human form. Once the flow of energy is cut-off and the residual energy in the body is used up, the human body ceases to exist.