Definition of a Hollow:

Hollows are departed human souls that have not been sent to Soul Society in due time, leading to their corruption. Once corrupted, they gain a skull-like mask, a drastically different physical appearance, and a very animalistic level of intelligence. Hollows crave other human souls, living or dead, to fill their empty hearts and ease their pain, starting with the persons it loved the most in its past life. They reside in Hueco Mundo when they're not scouring the human world for souls to consume. They will continue to search for and consume souls until they are slain by a Soul Reaper, at which point their souls are purified and they are allowed to pass on to Soul Society.

Some hollows have an unusually strong thirst for souls which leads them to consume other hollows. These hollows gather naturally, fighting to the death, and finally merging with one another, evolving into stronger and more intelligent forms known as menos (メノス menosu?, Spanish for "minus" or "less"). Within the menos, there are three levels of evolution:

The first of these is the gillian (最下大虚（ギリアン） girian?, Japanese for "least of the great hollows"), large and powerful but possessing very low intelligence and no individuality, the result of the merging of hundreds of hollows.

In the rare case a gillian retains its consciousness, it can continue to consume other gillians and eventually evolve into an adjuchas (中級大虚（アジューカス） ajūkasu?, Japanese for "intermediate great hollow"), gaining a smaller body and superior strength, speed and intelligence, compared to their previous form.

From there, some adjuchas have the potential to evolve into a vasto lorde (最上大虚（ヴァストローデ） vasuto rōde?, Japanese for "Great king of the Hollows"). Vasto lorde are more humanoid in size and appearance and possess battle strength comparable or superior to a Soul Reaper captain, to the point where captain Tōshirō Hitsugaya speculates that Soul Society would be doomed if Sōsuke Aizen managed to get ten vasto lorde under his command.

As hollows evolve they gain access to unique skills and abilities, fueled by the souls they have consumed. The most basic of these abilities is Cero (虚閃（セロ） Sero?, Japanese for "hollow flash," Spanish for "zero", labeled as "Doom Blast" in the Viz manga), high-powered energy blasts that can be fired from various body parts, such as the mouth or hand. There are at least two additional variants of the cero, though less commonly used.

Almost all antagonists in Bleach are, by definition, categorized as hollows. The following are those hollows that have not attained the further classification of arrancar. Many of the hollows shown in the series have few, if any, speaking lines and are killed shortly after their introduction.

Arrancar

Arrancar (アランカル arankaru?, Spanish for "to tear off") are hollows that have had their masks almost completely removed and have gained the powers of a Soul Reaper. In the process, they gain a human appearance and strength, speed and intelligence beyond the level of a simple hollow as well as hierro (鋼皮(イエロ) iero?, Japanese for ”steel skin,” Spanish for ”iron”), skin so hard they can fight against opponents with swords bare-handed. Arrancar become major antagonists during the second half of the series as they start being led by Sōsuke Aizen who uses the Hōgyoku to create and perfect the arrancar. According to Tite Kubo, many of the arrancar are named after real life architects and designers.

Like Soul Reapers, arrancar possess zanpakutō (斬魄刀? lit. soul-cutting sword). Unlike Soul Reapers, whose zanpakutō are manifestations of their owner's soul, arrancar's zanpakutō are manifestations of their former hollow selves. With occasional exceptions, an arrancar's zanpakutō resembles a regular sword in its sealed state. When released through a process called resurrección (帰刃（レスレクシオン） resurekushion?, Japanese for "returning blade," Spanish for "resurrection"), part of their original hollow form is restored, changing their appearance, giving them greater strength, and granting them access to their original abilities.

The Espada (十刃（エスパーダ） Esupāda?, Japanese for "ten blades," Spanish for "sword") are the ten strongest arrancar in Sōsuke Aizen's army. They are ranked in order of their strength. The members are listed according to their known rank. The first three hold the top three ranks although their specific ranks haven't been revealed, and as such are listed in order of introduction.

|| copied from Wikipedia ||

::Ranking::

Arrancars number 1-10 are ranked according to strength, with Arrancar 1 (Espada 1) the strongest and Arrancar 10 (Espada 10) the weakest from the ten. Arrancar 11-100 || I believe; correct me if I’m wrong || are numbered according to their births, with Arrancar 11 being the oldest and Arrancar 100 the youngest. Arrancars with numbers higher than 100 are Privaron Espada, or ‘used-to-be Espada’ who were overranked and given a new number.