Rudras are forms and followers of the god Rudra-Shiva and make eleven of the Thirty-three gods in the Hindu pantheon. They are at times identified with the Maruts - sons of Rudra; while at other times, considered distinct from them.
While the Vamana Purana describes Rudras as the sons of Kashyapa and Aditi, Maruts are described distinct from the Rudras as 49 sons of Diti, sister of Aditi, and attendants of Indra, rather than Rudra.
The Ramayana tells they are eleven of the 33 children of the sage Kashyapa and his wife Aditi, along with the 12 Adityas, 8 Vasus and 2 Ashvins, constituting the Thirty-three gods. The Vamana Purana describes the Rudras as the sons of Kashyapa and Aditi. The Matsya Purana notes that Surabhi - the mother of all cows and the "cow of plenty" - was the consort of Brahma and their union produced the eleven Rudras. Here they are named
The Harivamsa, an appendix of the Mahabharata, makes Kashyapa and Surabhi - here, portrayed as his wife - the parents of the Rudras. In another instance in the Mahabharata, it is Dharma (possibly identified with Yama) who is the father of the Rudras and the Maruts.
The Vishnu Purana narrates that Rudra - here identified with Shiva - was born from the anger of the creator-god Brahma. The furious Rudra was in Ardhanari form, half his body was male and other half female. He divided himself into two: the male and female. The male form then split itself into eleven, forming the eleven Rudras. Some of them were white and gentle; while others were dark and fierce. They are called:
From the woman were born the eleven Rudranis who became wives of the Rudras. They are:
Brahma allotted to the Rudras the eleven positions of the heart and the five sensory organs, the five organs of action and the mind. Other Puranas call them Aja, Ekapada (Ekapat), Ahirbudhnya, Tvasta, Rudra, Hara, Sambhu, Tryambaka, Aparajita, Isana and Tribhuvana.
In one instance in the epic Mahabharata, the Rudras are eleven in number and are named:
While Kapalin is described the foremost of Rudras here, in the Bhagavad Gita it is Sankara who is considered the greatest of the Rudras. Both Kapalin and Sankara are epithets of Shiva. In another instance, they are described as sons of Tvastr and named:
In Bhagavata Purana Canto 6 Chapter 6 the eleven Rudras are said to be the children of Sar?p? and Bh?ta. Sar?p? was a daughter of Daksa. The names of the eleven Rudras given in Canto 6 Chapter 6 Verse 17-18 are:
The Matsya Purana mentions the ferocious eleven Rudras - named:
Aiding God Vishnu in his fight against the demons. They wear lion-skins, matted-hair and serpents around their necks. They have yellow throats, hold tridents and skulls and have the crescent moon on their foreheads. Together headed by Kapali, they slay the elephant demon Gajasura.
In Vedic mythology, Rudras are described as loyal companions of Rudra, who later was identified with Shiva. They are considered as friends, messengers and aspects of Rudra. They are fearful in nature. The Satapatha Brahmana mentions that Rudra is the prince, while Rudras are his subjects. They are considered as attendants of Shiva in later mythology.
The Rig Veda and the Krishna Yajur Veda  makes the Rudras the gods of the middle world, situated between earth and heaven i.e. the atmosphere. As wind-gods, the Rudras represent the life-breath. In the Brihadaranyaka Upanishad, the eleven Rudras are represented by ten vital energies (rudra-prana) in the body and the eleventh one being the ?tman (the soul).
The Rudras are said to preside over the second stage of creation and the intermediary stage of life. They govern the second ritual of sacrifice, the mid-day offering and the second stage of life - from the 24th to the 68 year of life. The Chandogya Upanishad prescribes that the Rudras be propitiated in case of sickness in this period and further says that they on departing the body become the cause of tears, the meaning of the name Rudra being the "ones who make cry". The Brihadaranyaka Upanishad explicitly states the fact that since the Rudras leaving the body - causing death - makes people cry, they are Rudras.
The Mahabharata describes the Rudras as companions of Indra, servants of Shiva and his son Skanda and companions of Yama, who is surrounded by them. They have immense power, wear golden necklaces and are "like lighting-illuminated clouds". The Bhagavata Purana prescribes the worship of the Rudras to gain virile power.
Some scholars believe that Rudras and Maruts were earlier distinct groups, Rudras being the true followers of Rudra and daivic (Godly) in nature. But poets of the Rigveda forced the Maruts to take the position of the Rudras in order to give status to the Vedic god Rudra. Later in post-Vedic literature like the epics and Puranas, Maruts were associated with Indra, while Rudras gained their former status as followers of Rudra, who had evolved into Shiva. However, other scholars disregard this theory and consider that originally Rudras and Maruts were identical. A theory suggests that slowly in the Vedas two classes of Maruts came into existence: the friendly and beneficent, and the roaring and turbulent; the latter grew into the distinct group of deities called the Rudras, who were associated only with the wild Rudra.
In the Marut Suktas (RV 1, 2, 5, 8) and Indra-Suktas (RV 1, 3, 8, 10) of the Rigveda (RV), the epithet "Rudras" - originating from the verb root rud or ru and meaning howlers, roarers or shouters - is used numerous times for the Maruts - identifying them with the Rudras even when associated with Indra, rather than Rudra. There are some hymns in the Rigveda (RV 2, 7, 8, 10) that explicitly distinguish between the Maruts and the Rudras.
Ashwatthama, the son of Guru Drona is the avatar of one of the eleven Rudras and he is one of the seven Chiranjivi or the immortal ones. Drona did many years of severe penance to please Lord Siva in order to obtain a son who possesses the same valiance as of Lord Siva. Ashwatthama, the powerful son of Drona, though known as the part incarnate of Rudra, was really born of the four parts of Yama(death), Rudra(destruction), Kama(love) and Krodh(anger). Just before Mahabharata war, Bhishma himself declared that it will be virtually impossible for anyone to kill or defeat Ashwatthama in war as he is the part incarnate of Rudra. Bhishma said when Ashwatthama becomes angry then it will be impossible to fight him as he becomes a second Siva. No one can handle his wrath and fury. The tragic death of Drona made Aswathama extremely angry(outraged) and these events led to the complete annihilation of Pandava lineage by the hands of Ashwatthama himself.