Krishna (Sanskrit: कृष्ण Kṛṣṇa in IAST, pronounced [ˈkr̩ʂɳə] ( listen)) is the eighth incarnation of Lord Vishnu in Hinduism. The name Krishna appears as the 57th and 550th name of Lord Vishnu in Vishnu Sahasranama of the Mahabharata, and is also listed in the 24 Keshava Namas of Lord Vishnu which are recited and praised at the beginning of all Vedic pujas. A puja is the ritualistic worship offered in Hinduism.
According to the Bhagavata Purana, which is a sattvic purana, Krishna is termed as Svayam Bhagavan since he was the purna-avatara or full incarnation of the Supreme God Vishnu. Krishna is often described and portrayed as an infant or young boy playing a flute as in the Bhagavata Purana, or as a youthful prince giving direction and guidance as in the Bhagavad Gita. The stories of Krishna appear across a broad spectrum ofHindu philosophical and theological traditions. They portray him in various perspectives: a God-child, a prankster, a model lover, a divine hero and the supreme being. The principal scriptures discussing Krishna's story are the Mahabharata, the Harivamsa, the Bhagavata Purana, and theVishnu Purana.
Puranic sources mention  Krishna's disappearance marks the end of Dvapara Yuga and the start of Kali Yuga (present age), which is dated to February 17/18, 3102 BCE. Worship of the deity Krishna, either in the form of Vasudeva, Bala Krishna or Gopala can be traced to as early as 4th century BC. Worship of Krishna as svayam bhagavan, or the supreme being, known as Krishnaism, arose in the Middle Ages in the context of the bhakti movement. From the 10th century AD, Krishna became a favorite subject in performing arts and regional traditions of devotion developed for forms of Krishna such as Jagannatha in Odisha, Vithoba in Maharashtra and Shrinathji in Rajasthan. Since the 1960s the worship of Krishna has also spread in the West, largely due to the International Society for Krishna Consciousness.
courtesy : http://en.wikipedia.org/wiki/Krishana