"Behold! it is revealed by Aiwass the minister of Hoor-paar-kraat." (AL.I.7)
The Book of the Law is the central holy book of Thelema. Aiwass is the figure who is said to have dictated the work to Aleister Crowley on the three days of the writing of The Book of the Law in 1904. Details of these events are given in The Equinox of the Gods chapter 7.
It is impossible to discuss Aiwass without referring to The Book of the Law for it is through that work that we learn the nature of Aiwass. It is certain that Liber AL is written in a style wholely uncharacteristic of Crowley. Careful study of the work has revealed that it is filled with many qabalistic keys and other mysteries, many of which are still unsolved today. These puzzles accompanied with the hurried pace of Crowley's pen make it almost certain that the work comes from some form of higher intelligence communicating the message of the new aeon through Crowley. That intelligence is Aiwass.
Aiwass claims to be the miniter of Hoor-paar-kraat, a god the ancient egyptians known as Harpocrates. Harpocrates is a form of the god Horus depicted as an innocent child. His mother is Nut, or Nuit in Liber AL. This atribution further emphasizes the role of Aiwass in delivering the word of the crowned and conquring child.
Aleister Crowley believed Aiwass to have been one of the Secret Chiefs of the A.'.A.'. Crowley also eventually considered Aiwass to be his own Holy Guardian Angel.