Soul

A Plus is the soul of a Human. It is neither a Hollow, nor a shinigami. Plus are usually binded to their real body by the Chain of Life. As long as both, Soul and Body, are still connected, the soul can return into the body after some time. If the Chain of Life gets cut, the Soul can't return into the Body and is dead. If a Plus is cut from it's host, it can get eaten by a Hollow, which turns it into one. There are two different kinds of Plus.

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 break him to Soul Society.

Jikabu-Ghost
A Jikabu-Soul is not able to leave the real world that easy because he regrets leaving something behind. For example, if he is bond to a place, he will stay at this place until his Chain of Life vanishs. These are hard to trace by the Shinigamis, and mostly become a Hollow.