A lot depends on whether you are talking science, psychology, sociology, religion or philosophy. Each discipline presents us with a different view of mind, brain, consciousness, spirit and soul. Some disciplines use these terms interchangeably while other assign specific meanings to certain terms.
For example, Sociology is interested as to why people actually believe in spirits, souls and religion. It doesn't actually investigate whether such things actually exist.
Science on the other hand doesn't actually accept the soul or spirit as some type of abstract entity or substance independent of the physical. From a scientific point of view we can say that, "to know how the brain works is to know how the soul works ". In the end science is only investigates physical things in order to turn up physical explanations.
I would assume that different religions tend to have different explanations as to what the soul or spirit actually is. The essence of such explanations is probably centred on the claim that the soul/spirit continues on an existence even after death.
Philosophy provides an interesting history in this area. The idea of soul and spirit was the basis for much philosophical discussions for a thousand years or more. Recent development in the field has largely done away with the idea of soul/spirit in favour of brain and mind identity theories. Soul/spirit has been largely replaced with theories of consciousness.
Tut