Hollows are disembodied spirits of humans that have lost their connection to the world and are lost to despair.
All hollows were at one point human souls, but rather then pass on into Soul Society the souls became detatched from their humanity to escape the pain of being caught between life and the spirit world. When this occurs the Chain of Fate on their chest collapses on itself. This causes a process which is referred to ass hollowfication, at which point the soul implodes in on itself then explodes the explosion causes the soul take on a monstrous form followed by the souls face being covered up by a white mask. No longer is there a Chain of Fate on the chest but rather a hole. Part of the reason Hollows have earned their name the other part being they are all hollow in feeling human emotions. And only seek to fulfil their desires which have been perverted due to the hollowfication proccess. Because hollows possess an emptiness that no earthly means can fill they seek to devour both living and deceased souls to fill in the void. The souls thagt are eaten are then empty themselves and become more hollows. Hollows forms range from simple oversized frogs to more complex demonic looking abominations. Just as hollows appearence range drastically so do their powers. From super strength, agility, hardness of skin, to complex moves such as cero. Also hollows have huge variations in power levels, some are basic others can be god like in riatsu such as the vasto lords.
Menos Grande-The Menos Grande, also called Gillian (meaning "lowest great hollow"), are an evolved form of hollow that are the lowest class of menos, but still far more powerful than normal hollow. Gillians are huge monstrous creatures, towering at least 2 - 6 stories in height on average. They all have white masks with long pointed noses. Their body either consists of or is without fail, constantly enshrouded in what appears to be a black cloak extending all the way to their feet. Around their neck is a row of white spikes. Their feet are large, even for their proportionate size, and they are also white and pointed and almost resemble a sorcerer-like shoe (pointed and curled at the front.) Thay also possess large tongues that are capable of shattering the masks of other gillian.
All gillian possess at least 3 standard powers. Teose are the cero, kumon, and negacion.
Though the gillian are certainly capable of being defeated by stronger opponents, they are not to be taken lightly and even the shinigami captains acknowledge their strength. Their power is on a whole different level than that of normal hollows and a single gillian is powerful enough to be a challenge for even a vice captain. What makes them even more formidable is that most often the gillian attack in numbers.
Adjuchas-Adjuchas are created when a Menos Grande acquires self awareness thus acquiring an identity among the countless hollows that make up its body. This is very rare indeed in order to retain the idividuality the Menos Grande must feed on its own kind or risk forever loosing its identity. until eventually a smaller more compressed body is made. Each Adjuchas is unique in appearence and no two are truly alike. What an Adjuchas looses in size it more then makes up for in power and speed, also due to its acquired intellegence it is more combative in nature and can esily adapt unlike its larger counter part. In Aizen's army these are the most common among Espada.
Vasto Lord- Not much is truly known about this class of hollow other then the fact that they are EXTREMELY rare as Aizen only managed to find four or three for his army. They are the final evolutionary step in hollows that can only be surpassed through Aizen's process of creating Arrancar. All Vasto Lord weild Tremendous power enough so that it is said that ten Vasto Lord would be enough to crush all of Soul society. Out of the steps of evolution towards hollows Vasto Lord most closely ressemble humans in size and in humanoid shape. So far only one Vasto Lord's Ressurection has been seen.
Arrancar- Arrancar are hollows that have been altered giving them both Shinigami powers and a humanoid type body. The process that does this removes a section of their hollow mask. Arrancar can range in power from a weak underling to an Arrancar made from a Vasto Lord or Adjuchas. When questioned why the hollows follow Aizen even though he is a shinigami they responded because he possesses no fear something the hollows feast on.