The Bhagavad Gita (translated from the Sanskrit means "Song of God" or "Divine Song") is actually 700 verses from the Mahabharata. It is often viewed as one of the most important elements of the epic, although it is often presented on its own. Knowledge of the rest of the Mahabharata is not necessary in order to enjoy or understand the Bhagavad Gita. Krishna, as the speaker of the Bhagavad Gita is called Bhagavan (the divine one), and the verses themselves, full of similes and metaphors are written in a poetic form that is traditionally chanted; hence the title. The Bhagavad Gita is regarded as a sacred text by the majority of Hindu traditions, and especially so by followers of Krishna.
Though it is not exactly clear when the Bhagavad Gita was composed, western historians assume a date somewhere between 500 and 50 BCE. There are, however, considerable questions remaining on the subject. Based on the differences in the poetic styles most scholars have agreed that the Bhagavad Gita was added to the Mahabharata at a later date.
The Gita brings together many schools of Hindu philosophy, including those of Vedic origin and the parallel Yogic traditions. For its religious depth, quintessential Upanishadic and Yogic philosophy and beauty of verse, the Bhagavad Gita is one of the most compelling and important texts of the Hindu tradition. It is considered by many as one of the world's greatest religious and spiritual scriptures. Although it is not strictly part of the 'canon' of Vedic writings, almost all Hindu traditions draw upon the Gita as authoritative.
Although the Bhagavad Gita in an ancient Yoga text, it has very little to do with the physical practice of yoga (asana). The Gita follows Prince Arjuna's quest for spiritual guidance, and the God Krishna's answers to his questions on how to realize his inner spirituality and take responsibility for his life and role in the world.
Krishna counsels Arjuna to practice karma yoga, (the yoga of service), raja yoga, (the yoga of meditation) jnana yoga, (the yoga of knowledge particularly self-knowledge), and bhakti yoga (the yoga of devotion).

The Bhagavad Gita

The story is essentially a conversation between Krishna and Prince Arjuna taking place prior to the battle of Kurukshetra, the major battle that precipitates the final conflict in the Mahabharata.  The discourse begins with Arjuna, confessing his doubt on the battlefield. Realizing that his enemies are his relatives, some are beloved friends, and others revered teachers, he turns to his charioteer and guide, Krishna, for advice.
Responding to Arjuna's confusion, Krishna explains to Arjuna his duties as a famous warrior and Prince and elaborates on a number of different Yogic and Vedantic philosophies, with examples and analogies. Krishna reveals his identity as the Supreme Being Himself (Bhagavan), blessing Arjuna with an awe-inspiring glimpse of His divine absolute form.
The main philosophical subject matter of the Bhagavad Gita is the explanation of the five the truths. The truth of Ishvara (The existence of a Supreme Controller) Jiva (The Soul) Prakrti (Matter) Karma (Action) and Kala (Time)
Krishna explains to Arjuna the concept of dharma, or universal harmony and duty. He begins with the tenet that the soul is eternal and immortal. Any 'death' on the battlefield would involve only the shedding of the body, but the soul is permanent. Arjuna's hesitation stems from a lack of right understanding of the 'nature of things,' the privileging of the unreal over the real. His fear and reluctance to understand the true meaning of the universe are impediments to the proper balancing of the universal dharmic order. Arjuna wishes to abandon the battle and to abstain from action but Krishna warns that without action, the cosmos would fall out of order and truth would be obscured.
Krishna then relates  the various Yoga processes, and understanding of the true nature of The Bhagavad Gita proposes that true enlightenment comes from growing beyond identification with the ego, the 'False Self', so that one can identify with the truth of the immortal self, (Atman). Through detachment from the material world, the Yogi is able to transcend illusory mortality and attachment to the material world and enter the realm of the Supreme.
Krishna does not advocate abandoning the responsibilities of the physical world however. One's life on earth must be lived in accordance with greater laws and truths, and one must embrace one's temporal duties whilst remaining mindful of a more timeless reality. Leading such a life in balance will lead to stability, happiness and, ultimately, enlightenment.
Finally, to demonstrate his divine nature, Krishna appears to Arjuna in his universal form for a split second and reveals he is both the ultimate essence of Being in the universe, and also its material body.