The Vedas refer to 33 koti (types) of Devatas, not 33 crore of Devatas. They are explained in Shatapatha Brahman and many other scriptures very clearly.
“Yasya Trayastrinshad Devaa Ange Sarve Samaahitaa, Skamma Tam Bruhi Katamah Swideva Sah” (Atharva Veda 10/7/13).
Which means: With God’s influence, these thirty-three (supporting Devatas) sustain the world.
In Brihadaranyaka Upanishad while discussing Brahman, Yajnavalkya is asked how many Gods are there. He says that there are three hundred and three, [then he says there are] three thousand and three Gods. When the question is repeated, he says thirty-three. When the question is again repeated, he says six. Finally, after several repetitions, he says one (BU 1/9/1).
The number thirty-three comes from the number of Vedic Gods explained by Yajnavalkya in Brihadaranyaka Upanishad—the eight Vasus, the eleven Rudras, the twelve Adityas, Indra and Prajapati (BU 1/9/2).
- Eight Vasus: Earth, Water, Fire, Air, Ether, Moon, Sun, and Star. They are called Vasus because they are abode of all that lives, moves or exists (also mentioned in Mahabharata 1/66/18).
- Eleven Rudras: The ten Pranas (Prana, Apana, Vyana, Samana, Udana, Nag, Kurma, Krikal, Devadutta and Dhananjaya) i.e. nervauric forces which live in the human body. The eleventh is the human soul. These are called Rudras because when they desert the body, it becomes dead and the relations of the deceased, consequently, begin to weep. Rudra means one who makes a person to weep (also mentioned in Harivamsha 13/51-52).
- Twelve Adityas: The twelve months of a year called Adityas, they cause the lapse of the term of existence of each object or being (also mentioned in Mahabharata 1/65/15-16).
- One Indra, which is also known as the (all-pervading) electricity as it is productive of great force.
- One Prajapati, also called the “Yajna” because it benefits mankind by the purification of air, water, rain and vegetables and because it aids the development of various arts, and in it the honor is accorded to the learned and the wise.
The master of these thirty-three Devatas is the Mahadeva or Ishwar who alone is to be worshipped as per the 14th Kanda of Shatapatha Brahman. — Hinduism and Sanatan Dharma, 17 April 2014