In Hinduism, Ādityas (Sanskrit: आदित्य Āditya, pronounced [aːdɪtjɐ]), meaning "of Aditi", refers to the offspring of the goddess Aditi and her husband the sage Kashyapa. The name Āditya, in the singular, is taken to refer to the sun god Surya.
The Bhagavata Purana lists a total of twelve Ādityas as Sun-gods. In each month of the year a different Āditya is said to shine. According to the Vaishnava tradition of Hinduism, each of these Ādityas is a different expression of the Supreme God Vishnu in the form of the Sun-God.
In the Rigveda, the Ādityas are the seven celestial deities, sons of Āditi,
The eighth Āditya (Mārtanda) was rejected by Aditi, thus leaving only seven sons. In the Yajurveda (Taittirīya Samhita), their number is given as eight, and the last one is believed to be Vivasvān. Hymn LXXII of the Rig Veda, Book 10, also confirms that there are nine Ādityas, the eighth one being Mārtanda, who is later revived as Vivasvān. 
So with her "seven sons," Aditi went forth to meet the earlier age. She brought Mārtanda thitherward away to spring to life and die again.
The Ādityas of the Rig Veda are "devas", a distinct class of gods and are different from other groups such as the Maruts, the Rbhus or the Viśve-devāḥ (although Mitra and Varuna are also associated with the latter). 
In each month of the year, it is a different Āditya who shines as the Sun-God. As Indra or Vishnu, Surya destroys the enemies of the gods. As Dhata, he creates living beings. As Parjanya, he showers down rain. As Tvashta, he lives in the trees and herbs. As Pusha, he makes foodgrains grow. As Aryama, he is in the wind. As Bhaga, he is in the body of all living beings. As Vivasvana, he is in fire and helps to cook food. As Amshumana, he is again in the wind. As Varuna, he is in the waters and as Mitra, he is in the moon and in the oceans.
The Āditya have been described in the Rig Veda as bright and pure as streams of water, free from all guile and falsehood, blameless, perfect.
This class of deities has been seen as upholding the movables and immovable Dharma. Ādityas are beneficent gods who act as protectors of all beings, who are provident and guard the world of spirits and protect the world. In the form of Mitra-Varuna, the Ādityas are true to the eternal Law and act as the exactors of debt.
In present-day usage in Sanskrit, the term Āditya has been made singular in contrast to Vedic Ādityas, and are being used synonymously with Surya, the Sun. The twelve Ādityas are believed to represent the twelve months in the calendar and the twelve aspects of Sun. Since they are twelve in number, they are referred as DvadashĀdityas.
The 12 Ādityas are basically the monthly suns which is the ancient word for the earth moon barycenter for lunar month.[clarification needed] These are also called the 12 purushas, pertaining to the 12 lunar months of the year. Here the months refer to the lunar months. In astronomy the lunar months with a solar sankranti are said to have an Āditya or purusha. The month without a sankranti is said to be neuter and is said to be an extra month or the intercalary lunar month.
The Vedas do not identify the Ādityas and there is no classification of the thirty-three gods, except for in the Yajurveda (7.19), which says there are eleven gods in heaven (light space), eleven gods in atmosphere (intermediate space), and eleven gods in earth (observer space). In the Satapatha Brahmana, the number of Ādityas is eight in some passages, and in other texts of the same Brahmana, twelve Ādityas are mentioned.  The list of 12 Ādityas is as follows:
Ādityas are responsible for proper functioning of the universe and in Hindu cosmology they are given lordship over celestial constellations, called nakshatras in Jyotish. Nakshatras are forces of universal intelligence which are intertwined with the birth-death cycle of life, identity of all created beings, events and day to day consciousness in our lives. Ādityas manage the Shakti of the nakshatras. Here are few examples.
This makes Vedic Ādityas not some conceptual, abstract, or mythological characters in a story book, but part of the visible cosmology and the everyday realities of our daily lives. We manifest their qualities in our lives and as such are part of the divine ourselves.
Vedas and Zoroastrian Avesta have a common name Ahura-Mazda, which may refer to some Vedic God (sometimes in Rigveda some demigods or devatas are worshipped as "asura", which in Zoroastrianism is Ahura-Mazda. See also: Vishnu sahasranama (Āditya: 39 aĀdityah, 563 aĀdityah - Son of Aditi). Ahura-Mazda is commonly considered a link between Avestan Zoroastrianism and Asuras of Vedic literature, however there is no one specifically called Ahura Mazda in the Vedas.
For evolutionary reasons Asuras and Devatas fought great battles. Ādityas, sons of Rishi Kashyap and Aditi always followed the guidance of Trimurti, or the Trinity of Brahma, Vishnu and Shiva and are responsible for proper functioning of the universe, Asuras challenged their authority at various occasions. Most significantly there are constant battles for the Elixir of Immortality, called Amrit, between the two. This could explain why Avestan Asura-Mazda advised his followers to stay away from Daivas or Vedic Devatas, calling them untrustworthy and unscrupulous shining beings to be avoided at all cost.
Devata] including Ādityas are considered benevolent, and worshiped in the Vedas. There are various types of Devatas in Hinduism and Buddhism, all of them are venerable.
Historically there was little difference between Asuras and Devatas during the times of Veda. Many of them were highly regarded, and comparable to necessary forces of nature. In post Vedic era especially in the narratives of Puranas many Asuras became synonymous with trouble makers, who come into conflict with Mahadev Shiva, Vishnu, Brahma and Indra wreaking havoc on civilizations. There are some famous Asuras-Devatas conflicts including Samudra Manthan regarding churning of the Ocean. There are some famous Asuras such as Vritra-Asur, Bana-Asur, and Bhasma-Asura who challenge Ādityas and specifically Indra, the king of Devatas.
Going by Sanskrit definitions Asura is opposite of Sura. Sura is anything that is in harmony, in tune with laws of nature, called eternal truth or Sanatan Dharam. A-Sura is a being or force of nature which is chaotic, disorderly, and out of tune.