due to the leader's influence, the close proximity of so many strong hollows, evolution rates have soared over the past thousand years. where before there may be as many as 10 vasto lords at any one time, now there are over twenty, maybe reaching into the thirties.
the leader has also mastered an art of artificial shinigamification, so any hollow wishing it can go to a lab and be forced to turn into an arrancar. weaker hollows produce weaker arrancar, and end their evolution, but some do go for a quick fix, and some seem to do it naturally more often. this phenomena has not been completely explained yet, but it is possibly close proximity to arrancar causes easier evolution to arrancar.
evolved hollows have authority over lower hollows, in hueco mundo, it is the strongest who is in charge, so adjuchas rule gillians, and gillians can force hollows to do their bidding, (if they retained their minds).
but even after evolution finishes, the espada are in charge. they hold their position through their might, to become one you simply have to beat the one who holds a position, most go for espada 10, because he is supposedly the weakest, as they are numbered in terms of strength.
beneath each espada is what is called a fraccion, to become a part of one you must be accepted by said espada, but most hollows recieve misisons from fraccion even if they are not aligned to one.
hollows. form varies widely, but all have a mask and a hole, the mask is made of white hollow bone, as do all hollow types, and can vary in shape. all hollows have a form based on some creature in someway. examples from the series are, shrieker/bat, acidwire/snake and grandfisher/hamster
gillians. the classical menos grande, they all look basically the same, and for almost all they have no mind, a few special ones retain some instinct and ability, but for the most part, they are dumber,slower, and heftier then their brethren
adjucha. most have alot more of the hollow bone than hollows, some covering their body, some just more of their body. again they tend to resemble some animal, grimmjow/panther di roy/worm
vasto lord. the final normal evolution. these are nearly perfectly humanoid, and sized, a prime example of one who may have been vasto lord is baragan.
arrancar, a hollow that has managed to remove his mask and gain shinigami powers. the stronger and more perfect the arrancar, generally the more humanoid and less mask remains they have on their face. the increase is not the same for all hollows, some may increase their power by 1000% some by 10%, it's just luck of the draw.
ressureccion. when arrancar call out their release phrase, they enter ressureccion. their release form reunites their current form, with their hollow form. examples
http://www.onemanga.com/Bleach/284/08/ hollow forms
one release
http://www.onemanga.com/Bleach/207/19/another release
http://www.onemanga.com/Bleach/208/17/last
http://www.onemanga.com/Bleach/204/11/a note on ressureccion, when a hollows form in ressureccion is damaged, it is damaged for good, unless they can heal.
http://www.onemanga.com/Bleach/258/10/
