To counter the rising popularity of Buddhism, which democratized the teachings of the Upanishads, the Vedic religion moved away from its sacrificial rituals and the monistic tendencies of the esoteric Upanishads and brought out personal theism in the form of divine stories (Puraanas). The paths of devotion (bhakti) and work (karma) were introduced and placed alongside knowledge (jnaana), as means to salvation.
Mahaayaana Buddhism represents the reaction to this movement of Vedic Braahmanism. The monastic tendencies of the earlier Buddhism gave place to a more accommodating religion, which appealed to the normal human heart. The Buddha was made a God and personal devotion to him encouraged. The Buddha is no preacher of penance and doesn't shut his eyes to the world on attaining nirvaana : "I would be a guard to them that have no protection, a guide unto the traveler, a ship, a well spring, a bridge for the seekers of that shore; I would be a lamp to such as need a lamp, a bed for the weary that need a bed, the very slave of such that need service".
It was also felt that the original missionary spirit of the Sangha was on the wane, with monks being more self centred, interested only in personal salvation. Hence the reclusive (arhat) ideal of the Sthaviravaadins was replaced with the Boddhisattva, who struggled and sacrificed to attain Buddhahood and would help in the salvation of all beings in the world. So while the Sthaviravaada with its monastic tendencies is condemned as Hinayaana or the "Lesser Vehicle", the Mahaayaana prides itself on being the "Greater Vehicle" to deliver all beings from samsaara.
Mahaayaana Buddhism gives positive ideas of God, soul and human destiny. It offers to all beings in all worlds salvation by faith and love as well as by knowledge. The Hinayaanists had refuted the Self and claimed reality to only the underlying elements (dharmas) of existence. The Mahaayaanists go one step further and deny reality to both the Self and the elements. They finds in their nirvaana, the One Reality, which is shunya or void, only in the sense that it's free from the limitations of every phase of the limited or contingent experience of which we have empirical knowledge.
Rebels from brahmin ranks like Ashvaghosa and Naagaarjuna themselves pioneered this movement in Buddhism. Probably due to this braahmanical influence Sanskrit replaced Paali as the lingua franca.
The Maadhyamaka and the Yogaacaara are the two speculative schools of the Mahaayaana.
The Mahaayaana considers the following as its scriptures : Astasaahasrikaapraajnapaaramitaa, Gandavyuha, Dashabhumiishvara, Samaadhiraaja, Lankaavataara, Saddharmapundarika, Tathaagataguhyaka, Lalitavistara and Suvarnaprabhaasa.
An abridged version of Praajnapaaramitaa is Naagaarjuna's Mahaayaana Sutra, from which he derives his own Maadhyamaka Shaastram.
The Lankaavataara Sutra is the source of the Yogaacaara views.