A Hollow that has removed its mask and has gained Shinigami-like powers
In the Bleach series there are many characters that are proving to be very powerful. One of these teams is called the Arrancar. Arrancar are Hollows who have given up their masks and used the Orb of Distortion to gain a Soul Reaper's powers. During the series Arrancar show up in many forms and have various power levels.
The first Arrancar that is revealed in the series is Grand Fisher after he comes back to earth to destroy Ichigo for having been defeated by him earlier. Grand Fisher looks like a giant human who is wearing his Hollow mask half off and has his Hollow body draped over himself. Grand Fisher is an enormous monster, but he is no where near as powerful as a full Arrancar. The first fully powered Arrancars that show up are Grimmjow Jaegerjaques, and Espada and five of the weaker Numero Arrancar. Most Arrancar look like Soul Reapers, but still have fragments of their masks on their heads. Since the Arrancar are suppose to be the Hollow equivalent to Soul Reapers they wear a uniform similar to the Soul Reapers, only they have various jackets and their uniforms are white. Most of the Arrancar carry a Zanpaku- to and display a Hollow's hole on their body. All the Arrancar have very independently defined personalities, but they are all very fierce and dangerous in battle.
There are several classes set up for the Arrancar, the strongest being the Espada, and the weakest being the Numero. Also in the classes is a list of Arrancar that were once Espada, but have lost their rank because of more powerful Arrancar coming into existence. These are called the Privaron. Also there is a class of Arrancar that basically servants for the Numero, much weaker than the other arrancars in comparison. These are the Fraccion.
The fraccion are weak and only exist for the sole purpose of serving the Numero. The number of Fraccion varies depending on the Numero and how much they feel that they need servants.
The Numero are the middle ground for the Arrancar ranks. They are weaker than the Privaron and Espada, but have never lost their rank. Some of the Numero are at a lieutenant level of power, but only when the lieutenant's power is sealed in the human world. Grimmjaw took a team of Numero to attack Ichigo and the others at the beginning of the Arrancar storyline, and proved them to be very powerful at first. Once the power limitation was removed from the Soul Reapers, however, the Numero proved to be at about the same level as a fifth seat or less. There are many more Numero than there are Espada but have the same abilities usually. All Numero's rank is determined by a two digit number.
The Privaron are the unfortunate few that were once Espada but were replaced. When Aizen brought the Orb of Distortion with him to make new Arrancar. The Privaron had their old one digit number replaced by a three digit number that identifies them as being stronger than Numero, but not as powerful as their replacements and former peers. So far there has been three Privaron shown, each in a ridiculous appearance and behavior. The Privaron were once at the top of the hierarchy of the Arrancar ranks but have fallen, now they will do anything to get back to the top of the list.
The Espada are the comparitive equivalents of a Soul Reaper Captain. The Espada are the top ten Arrancar and have a number to identify their rank tattooed on their body. The Espada are very powerful and are former Vasto Lorde and Adjuchas, with the exception of one who was a Gillain class Menos Grande. The Espada are each very different in their power according to their rank too. The difference between the number one ranked Espada and the tenth ranked Espada is very large. It is mentioned that the Espada are feared to be even more powerful than a Captain level Soul Reaper. Many of the Epsada are replacements for former Espada that were weaker than the ones that were created by Aizen.