A Soul is the incorporeal and immortal essence of something, animate or inanimate. It is made of mana, a supernatural force believed to occupy all aspects of the universe.
In a person, a soul is a manifestation of their personality, psyche and abilities. People with special types of soul, thought to have been "blessed by God", can convert their mana into a tangible energy.
A soul appears as a small ball of light. Different people's souls are different colours, and a soul is distinguishable by its colour. A soul is also distinguishable by its number. 
Types of Souls
In the 07-Ghost universe, someone who possesses a pure soul is considered to be good. Pure souls are rare and widely sought after by beings of darkness as they are more "delicious" than tainted souls. Pure souls are capable of many things: such as being able to comfort people with their presence; eradicate darkness from the souls of those around them; and recognize when something has been infected with negative feelings.
Pure souls are also more sensitive to darkness than other people.
A pure soul usually shines with a bright light. The more pure the soul, the brighter it shines.
Tainted souls are commonly found amongst Warsfeil and Wars; the servants of the shinigami Verloren. A tainted soul has a 'disgusting' taste, and causes nausea to those who devour it that 'even smoking' cannot remove.
As shown in the case of Lady Marie, some souls may be trapped and prevented from returning to Heaven. However, Verloren's Scythe can set these souls free.