Soul

Pluses (Wholes in the official English editions) are the benign ghosts in the Bleach universe. They are 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 (Konsō) 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 a spirit who 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 and refuse to pass on to soul society.

Hollowfication
All plus souls have a chain connected to their chest known as the Chain of Fate, as time passes by and they aren't sent to Soul Society via Soul Burial this chain erodes by growing small mouths that eat away at itself. This process is painful and starts out slow but each erosion becomes more violent then the last until finally the chain is completly gone. At this point the souls body disperses into spirit particals and then reforms nearby as a hollow, the body usually forming first followed by the mask unless the soul resists in which case the mask might form first.

There are other ways to force hollowfication such as prying open the hole in a plus' chest by force as shown by Don Kon'onji. There are also theories that when a hollow eats a puls they choose to either turn it into a hollow or absorb it as energy, however this has no proof behind it. Some may say Inoue Orihime's brother, Sora, can be used as an example of this, however this was never shown or mentioned in the manga. In the anime while he is swarmed by hollow they all back away within a matter of seconds to show Sora's body fully intact just before he undergoes hollowfication making it less likely they ate him and more likely that they merely removed his Chain of Fate and opened the hole in his chest by force.

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