Hollows
Hollows are former Pluses (deceased human souls) that lose their hearts to despair or remain in the real world for too long. Non-human spirits can also transform into Hollows. Any spirit that is not guided to the Soul Society by a Shinigami via Soul Burial may eventually descend into a Hollow.
The majority of ghosts peacefully move on to the Soul Society after their deaths. Some, however, stay in the living world to fulfill certain goals, such as looking after a loved one, protecting a particular location, or pursuing a grudge from beyond the grave. However, with only limited abilities to interact with the living, and by definition tied to their goals, many ghosts find they are unable to cope when their loved ones move on or old enemies forget them. At this point, some will become dangerously obsessed with fulfilling those goals, even if the means to fulfil them renders the soul empty: for example, killing anyone who comes near a loved one, in order to keep them from hurting that person. In doing so, the ghost warps into a Hollow and, ironically, often makes the focus of their obsession into their first victim, such as Orihime Inoue's brother, Sora, does. This can be prevented if a Shinigami gives the soul a Soul Burial before it descends into a Hollow.
The process by which a soul becomes a Hollow usually takes a year, reflected by the length of the soul's Chain of Fate, which resides in the centre of their chest. The Chain of Fate slowly corrodes over time, reflecting the degradation of the soul's ties to the living world. The process can be accelerated under certain conditions, such as being attacked by a Hollow, giving into extreme despair or through the use of certain spiritual powers. When the chain completely decays, they become a Hollow when a hole opens up on their chest, where the chain was attached (signifying that they have lost their heart).
Once a soul becomes a Hollow, it is driven by an eternal and insatiable hunger to devour other souls, living or dead, to fill the void caused by its descent. While their former obsession often becomes their first target, most Hollows will eventually become mindless creatures concerned only with finding the most potent souls to devour. Stronger and more evil Hollows are often able to retain some of their intelligence in Hollow form, making them more dangerous than the average Hollow. While most Hollows are evil, or at the very least mindless, at least a few are not, as is the case with Nel Tu and her Hollow companions. They are also at least partly capable of camaraderie and loyalty, as seen with Grimmjow Jaegerjaquez and his companions while they were still Hollows: Grimmjow's subordinates sacrificed parts of their bodies, unasked, for the sake of Grimmjow's growth
Credit to Okami