Hollows are the spirits of humans who have lost their connection to the human world.
A person becomes a Hollow when their Chain of Fate is either removed forcefully or falls off when the hole in their chest completes itself.
After becoming a Hollow a person begins to hunt down people that they either know or have strong spiritual powers.
A Hollow consumes souls to fill in the hole that was created when they changed over.
Most Hollows can have their souls cleansed and then move on to the Soul Society after they have had a Zanpakuto cut through their masks.
Other Hollows who have committed great crimes against humans while they were still alive are banished to Hell.
Hollows wear bone- like masks that cover their face, hiding who they are from anyone that sees them.
These masks also protect their heads, which is also their weakness.
Hollows are also more animal looking in their appearance, while still holding some resemblance to a humanoid.
Stronger Hollows will retain their personality after they have changed from their spirit form, while weaker Hollows will be very primal in their behavior.
A truly powerful Hollow may evolve into a Menos Grande level Hollow.
Not all Hollows are empty minded monsters that converge and kill every strong spirit they come across.
Some Hollows are very intelligent and use their individual powers to attack with.
Others even use other Hollows to help them attack and destroy their opponents.
The Menos Grande are a Hollow that is made when a large number of smaller Hollows gather together to make one larger Hollow.
As a Menos Grande the power of the Hollows is amplified and usually requires the imperial forces of the Soul Reapers to destroy them.
A Menos Grande Grande are usually very unintelligent, but all look very similar to one another.
The Gillian Usually is covered from head down in a black shroud, have spikes around their necks and a white mask with a long needle- like nose.
Another form of the Menos Grande, yet very different from their Gillian counterparts.
The Adjuchas are fewer in number than the Gillian, but they have higher intelligence and powers.
If the Gillian are the grunts of the Menos, then the Adjuchas would be more like the Noncomissioned Officers of the Grandes.
The Adjuchas have only been shown as a silhouette so far so all information on them is based on conversations during the series.
The third class of the Menos Grande is the Vasto Lorde. This level of Hollow is legendary: in fact none have ever been reported as being seen, despite this the rumors of them persist.
Even Aizen was searching for Vastro Lordes by sending out his Arrancars: who in turn could not find them. Grimmjow Jaegerjaques aspired to become a Vastro Lorde.
However it was a level neither he, nor his fraccion could ever achieve through centuries of trying.
While they have not yet been seen they have been the subject of a conversation where Toshiro Hitsugaya claims that they are capable of destroying the average Soul Reaper captain.
Being the final form of the Grandes, The Vasto Lorde would logically be the most powerful and have the few in numbers.
During his final fight with Ulquiorra Cifer, the Curata Espada, Ichigo Kurosaki had apparently been killed after sustaining too much damage during the fight.
Just as Ulquiorra was about to kill Uryu and Orihime, Orihime screamed for Ichigo to save them.
Orihime's cries for help were heard and answered in the form of a resurrected Ichigo Kurosaki...the only difference was that he had mutated into a Hollow that looked precisely like a Vasto Lorde.
Ulquiorra demanded to know what Ichigo had become but received no answers...only pain and suffering as the feral Ichigo tore through Ulquiorra with his Zanpaktou.
Though Ulquiorra did his best to fend the Vasto Lorde off, it eventually overwhelmed him and fired a Cero at point blank range.
When it seemed like the fight was over and The Vasto Lorde had ripped Ulquiorra to shreds, Uryu tried to calm him down only to end up having Ichigo's Zanpaktou driven through his body.
The Vasto Lorde pinned Uryu's body against a rock face and turned in Orihime's direction, stating in a garbled form of speech that he would forever protect her from all threats.
After stating it's intentions The Vasto Lorde turned back to Uryu's injured body and prepared to fire another Cero, interestingly enough Uryu was saved at the last minute by Ulquiorra, whose body was still partially regenerating.
Ulquiorra was able to break off The Vasto Lorde's mask and send it's body into shock, which seemingly killed Ichigo once more.
However as Ichigo's corpse began to revert back to it's previous state he underwent an instantaneous regeneration cycle and was restored to life once more but not as a Vasto Lorde, however Ichigo did retain all memories of what had happened during his time as The Vasto Lorde.
Since the day of Ulquiorra's death, Ichigo has not transformed back into The Vasto Lorde. Whether he is still even capable of doing so remains uncertain.
Later in the series several of the Hollows convert themselves over to Arrancars, a Hollow who sealed his powers and learned some Soul Reaper abilities.
An Arrancar evolution can only be done with Aizen and the Orb of Distortion. The arrancar have resurrection just like bankai of shinigamis although ulquiorra have shown to have a second resurrection.
It is possible to have a shinigami to have to power of a hollow. Only a vizard can have sufficient talent to surpass both hollows and shinigami. However, the process to be a vizard is dangerous. Also, when the shinigami calls upon the power of hollow, a mask will appear over their face and their personality will change.