The Bhagavad-Gita is known as one of the most fundamental texts that form the basis of Hinduism, which has become one of the world's great religions. At the same time, however, the idea of Hinduism as a religion is a bit of a misnomer. As the Gita suggests, Hinduism is a way of live -- a philosophy -- that parents are expected to teach their children, creating an oral tradition, supported by the passing of the Vedic texts from generation to generation. (Indeed, many Hindus say that you cannot 'convert' to Hinduism. That you are simply born a Hindu or you are not.)
The central tenet at the core of Hinduism is the idea of karma, or debt created by action. Through one's life, one is constantly performing action that takes us towards self-realization or away from it. If we are moving towards self-realization, then we are working off our karma -- or debt from past lives. If we are moving away from self-realization, then we are simply accumulating karmic debt which will have to be worked off in subsequent lives. Practically speaking, many Hindus refer to a moment in life where the switch literally 'flips' -- where they go from accumulating karmic debt to the long journey of working it off. Hinduism believes that karmic debt usually cannot be worked off entirely in one lifetime. That one has to be constantly reborn in the samsaric cycle of birth and death in order to ultimately dissolve all karma and be freed from earthly life -- that is, to achieve moksha, or liberation.
Hinduism and Buddhism both share in common principles of living attributed to the teachings of the Buddha -- often referred to as Siddhartha, who found enlightenment through casting off worldly possessions and attachments in order to find nirvana. But Hinduism evolved to preach not asceticism or renunciation, but rather a more complicated form of enlightenment - namely yoga. Whereas Buddhism has spawned movements such as 'Zen,' encouraging the cessation of action to find mindfulness, Hinduism asks its followers to be 'yogis,' or 'skillful in action.' What Hinduism says is that by practicing meditation and being mindful during everyday actions, eventually we can find meditation no matter what we do -- we can meditatively work at the office, we can meditatively play basketball, we can meditatively survive morning rush hour. We'll come to live in the present, free from the distractions of the mind, without the fear of having to give up the world and its attachments.
Krishna, one of the central figures of polytheistic Hinduism which encourages the worship of many gods, is front and center in the Gita. Krishna is technically an avatar of Vishnu, one of the trinity of Hindu deities (with Brahma and Siva), but in the Gita he assumes complete omnipotence. What Krishna best represents in Hinduism is the idea of 'lila,' or life as a form of God's play. Having Krishna engage in conversation with Arjuna is suited for the Gita because Krishna can best convey the metaphor of man as God's puppet. We are here to fulfill our duty in God's larger plan -- our dharma, as Hindus term it -- and we cannot resist it, we should not despite it, and we will eventually, no matter how many lifetimes it takes, come to love it.
Our ultimate goal, and one that we will likely never reach is not just nirvana, but Darshan, as Hindus call it -- which is the literal beholding of the divine. Arjuna has this rare, perhaps impossible moment, when Krishna reveals his full powers, but Darshan remains a step beyond self-realization, offered perhaps only to those who achieve moksha in their lifetime.