Soul

Benign ghosts in Bleach are known as pluses (Wholes in the official English editions). A plus is the spirit of a person who has died. A chain, known as the Chain of Fate, protrudes from the chest and binds the plus to a location, object or person that they felt close to in life. The soul can move about freely if the chain is broken, but this also causes the chain to corrode. Normally, pluses are sent to Soul Society by Shinigami in a ritual called soul burial (Konso) before this corrosion becomes significant. If the Chain of Fate is corroded entirely before a soul burial can be performed, a Hollow hole will form in the chest of the soul where the chain was once anchored. Such souls are driven mad and become evil ghosts known as Hollows. If the Chain of Fate is torn out deliberately, this hastens the process. There are two different kinds of Pluses known.

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

Jibaku-Plus
A Jibaku-Soul or Earth-bound Spirit is not able to leave the real world that easily because he or she regrets leaving something behind. For example, if the soul is bound 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, so most of them become a Hollow.

Hollowfication
When a plus is eaten by a hollow, they either become a part of it, adding their energy to it's power, or like Orihime's brother, Sora Inoue, become a hollow themselves.

For more information on the degradation of a Plus' Chain of Fate, see Encroachment, as this is the slower process of becoming a hollow.