There is not clear information about the form of revelation in the Quran. We receive information about the issue from the Prophet. We can list those forms as follows:

Type I : True dreams. The following is stated in a hadith reported from Hazrat Aisha: “The first revelation that came to the Messenger of Allah (pbuh) was the true dream. Each dream the Messenger of Allah saw turned out to be true.”80

Type II: The angels of revelation brought the revelation to his heart...

Type III: Gabriel, the angel of revelation, addressed the Messenger of Allah disguised as a human being. It was the easiest type. 

Type IV: The angel of revelation came down with a sound like a gong or bell and delivered the revelation. It was the hardest type for the Prophet. The angel was not visible. When the revelation ended, the Prophet remembered the words. That type of revelation was usually related to the verses that expressed warning and threat.

Type V: Gabriel came down in his own appearance. That kind of revelation occurred twice: One in Mount Hira, the other in the Ascension (Miraj)...

Type VI: The revelation that was received directly from Allah without the intermediary of the angel in the Ascension. The Messenger of Allah received the revelation about the five daily prayers like that. In this type, the Prophet did not see Allah; he only heard Him.

Type VII: The revelation that the Prophet received from Allah by seeing Him.81 Only Hazrat Muhammad received this kind of revelation; he saw God Almighty in the Ascension.82

80  See: Tajrid Trns., I, 10-11

81  For detailed information about wahy, see: Tajrid trns., 1/3-7

82  For detailed information about the Ascension, see: Sözler, 593-621; Mektûbat, 281-285.