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-Menos Grande large giant like hollows wearing black cloaks that are formed when a large number of hollows who have abnormally high hunger start to perform cannibalism and gather in certain areas' of Los Noches. when this occurs, the desire of hunger spreads around connecting the hollows through riatsu until eventually all the cannibalistic hollows merge into a single being Menos Grande. Menos Grande have no personality due to the fact that they are composed of a large number of hollows they are ill in intelligence. No different then beast but they are able to perform Cero which is a blast of pure concentrated riatsu.
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.