Menos
Though most Hollows are simply transformed Human souls, a sub-group of Hollows known as Menos (menosu; Spanish for "Minus" or "Less"), exist. Menos are conglomerations of hundreds of normal Hollows, resulting in a single composite entity of far greater strength than any normal Hollow.

Menos are created when the void within an ordinary Hollow's heart becomes so substantial that Human souls are incapable of sustaining it, at which point it begins devouring fellow Hollows. These cannibal Hollows are attracted to one another, and a mass inter-devouring follows, resulting in a Gillian.[3] Following this transformation, the Gillian can evolve twice more, first into an Adjuchas and finally into a Vasto Lorde, provided certain conditions are met. Menos evolution is not only contingent upon the Menos eating other Hollows, but also upon the Menos' ability to avoid being eaten. If another Hollow succeeds in devouring even a part of their body, evolution becomes impossible for the one which was wounded this way. Menos-class Hollows are considered so dangerous that Soul Society sends only highly trained Shinigami teams to handle them.[2] These incursions seem to be rare, however, as Menos rarely leave Hueco Mundo unless attracted by large sources of spiritual power or encouraged to by other forces.

Gillian
Gillian (girian; Japanese for "Lowest Great Hollow"), also called Menos Grande (menosu gurande; Spanish for "Less Big" or "Big Minus", Japanese for "Great Hollow") are the weakest class of Menos and the first stage of Menos evolution. They are giant in comparison to other Hollows, dwarfing even the largest standard Hollows by at least the size of a two-story building, but lack agility and intelligence. They also have long tongues capable of breaking a fellow Hollow's mask. While a Gillian-class Menos is quite powerful, they are still much weaker than a Shinigami lieutenant. According to Tōshirō Hitsugaya, they are comparable to "foot-soldiers", reflected in their uniform appearance. In Soul Society, Menos Grande are shown in textbooks discussing Menos. Up to the point when Rukia meets Ichigo, she has only seen them in such textbooks.

Though Gillians usually have no personality to speak of, which makes them dangerous since they have no mind of their own, one of the many Hollows that comprise it might gain control over the new entity, reflected in the unique mask it wears in comparison to a normal Gillian. Only these Gillian variants have the potential to evolve into Adjuchas-class Menos. The evolution is facilitated by the continued cannibalization of fellow Hollows, even other Gillians.

Aaroniero Arruruerie is the only Gillian to have become as powerful as an Adjuchas without actually changing into one. This happened due to his unique ability of completely absorbing other Hollows, thus gaining all of their powers and abilities. Because of that, having the potential to become near-omnipotent, he was welcomed into the ranks of the Espada. Fortunately, though, before approaching anything near omnipotence, he was killed by Rukia Kuchiki.

Adjuchas
Adjuchas (ajūkasu; Japanese for "Intermediate Great Hollow") are the middle class of Menos in terms of power, and the second stage of Menos evolution. Adjuchas can vary radically in appearance; anywhere from bulky humanoids to the jaguar-like form of Grimmjow Jaegerjaquez. The are typically much smaller than Gillians, but significantly-larger than a Human. They are also-much smarter than the Gillian-class, and have the power to challenge a captain. It is also likely they could fight evenly with at least a Shinigami lieutenant. They watch over the many Gillian-class Menos and in the anime are also capable of controlling them, or at the very-least, those put in charge by Sōsuke Aizen.

Adjuchas-class Menos are much smaller in population compared to the Gillian class, due to the rarity of any one Hollow being capable of overwhelming the many other Hollows that comprise the body of a Gillian. Furthermore, the Adjuchas must continue to devour Hollows, or its mind will be absorbed by the other Hollows comprising its form and it will revert back into a Gillian. The subsequent lack of an individual will also preclude the reverted Gillian from becoming an Adjuchas again.

Vasto Lorde
Vasto Lorde (vasuto rōde; Japanese for "Highest Great Hollow") are the most-powerful class of Menos, and the final stage in the Menos evolutionary line. They appear to be about the size of a Human and apparently appear to be completely humanoid. They are said to surpass an average Captain-Class Shinigami in battle abilities.

Vasto Lorde are the least common form of Menos, but the exact population is unknown. Tōshirō stated that only a few are known to exist, and are believed to reside within the very deepest parts of Hueco Mundo. Apart from the need to cannibalize even more Hollows, any special requirements for the creation of a Vasto Lorde are unknown.

Tōshirō claims that at least ten of them, under Sōsuke Aizen's control, would be enough to defeat all of Soul Society. This is no small coincidence, as Aizen himself states to Gin Ichimaru that he needs to gather enough Vasto Lordes to complete his ten-member Espada. It seems that Aizen has important plans with them, but the only details revealed about those plans are the mere wish to gather all of them together.

From Bleach Wikia