Who is Krishna?
Krishna was one of the Hindu gods. He was actually just one of the many forms of the god Vishnu. Krishna was thought of as one reincarnation of Vishnu. Krishna doesn't appear in Indian stories before about 300 BC, and probably he and Vishnu were both newer gods that came into Hinduism around the time of the Mauryan Empire, as the older gods like Indra, with their demands for animal sacrifice, started to seem old-fashioned.
Krishna appears in many complicated stories. Sometimes he is a child, and sometimes a flute-player who calls people's souls, and sometimes he is involved in love affairs with Hindu goddesses, and sometimes he is a great leader.
Krishna's most famous story is in the Bhagavad Gita, written about 300 BC, where Krishna, disguised as a charioteer, tells the young prince Arjuna how to act according to his dharma, and generally what is good and what is bad.
Learn by doing: visit an art museum and look at Indian art about Krishna
More about Vishnu