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The celestial beings are called Sur (or Devtaa) and their antagonists are A-Sur. There are many Sur beings living in Heavens and netherworlds. To name a few, they are Urag, Garud, Gandharv, Kimpurush, Siddh, Saadhya, Vidyaadhar, Chaaran, Apsaraa, Yaksha, Guhyak, Bhoot, Khecharaa and the like. Of them Gandharv are amiable beings for their expertise in performing arts and Apsaraa for their beauty, singing and dancing. 

The Apsaraa beings are the divine courtesans and among them are four Apsaraa who are of high order. They are Rambhaa, Urvashee, Menakaa and Tilottamaa. These four are instrumental to Lord Indra, to incite and allure sages from their austere penance, lest they may win over the Lordship on heavens from Lord Indra. 

The present Gandharv, namely Tumburu, in his courting with Rambhaa belated his services to Kuber, the Chief Divinity for Wealth Management. Hence Kuber cursed this Gandharv to become the Raakshas called Viraadh, but yet this Tumburu alias Viraadh is the devotee of Goddess Lakshmee. The antagonists of Sur are A-Sur beings and all sorts of demons, monsters, etc., come under this category. They are not devils or Satan's stooges. They equal the capabilities of Sur beings. 

Prof. Ranade observes that "Dr. R.G. Bhandarkar, in an important article in the B.Br.A.S. Journal makes the following interesting suggestion. The Sanskrit equivalent of the word demon viz., 'Asurya' may here refer to the 'Assyrian' country. 'Assyrian and 'Asuryan' being philologically identical, the 'y' and the 'u' being interchangeable as in Greek..."

[Taken from Valmiki Ramayan 3/4/9]