Soul

A Plus is the soul of a Human. It is neither a Hollow, nor a Shinigami. Plus'es are usually bound to their material body by the Chain of Fate. As long as both soul and body are still connected, the soul can return into the body after some time. If the Chain of Fate gets cut, the Soul can't return into the body and is dead. If a Plus is cut from its host, it can get eaten by a Hollow, thus it becomes part of it. There are two different kinds of Pluses known.

Normal-Ghost
A normal Plus. He leaves the Real world in his death without regrets and without losing anything, and waits for a Shinigami to transfer him to Soul Society via Soul Burial.

Jibaku-Ghost
A Jibaku-Soul or Earth-bound Spirit is not able to leave the real world that easy because he or she regrets leaving something behind. For example, if the soul is bond to a place, he or she will stay at this place until his or her Chain of Fate completely corrodes. These souls are hard to trace by the Shinigami, thus most of them become a Hollow.