The Gita appears at the point in the Mahabharata where a war is about to take place. The royal family is divided over who should rule in the next generation: the rightful Pandavas or their cousins, the Kauravas.
Arjun is the most talented fighter in the Pandava army, and he is preparing for battle. His charioteer is Lord Krishna, and Krishna is a manifestation of God.
While facing his cousins in the battle, Arjun feels disheartened and discouraged. He does not want to wage war against his own family, even if they are ignorant and driven by a lust for power.
The Gita begins here, when Krishna begins to counsel Arjuna. The dialogue becomes very deep as Krishna explains the nature of the imperishable soul, reincarnation, the law of karma, the forces of nature and the way to find union with God (yoga).
The main points of the Gita are these:
1. Every person is born to fulfil certain responsibilities according to their nature and place in the interdependent web of creation.
2. Death is an illusion. The soul passes from one body to the next until it fulfils its karma in the cycle of creation.
3. A yogi (one united with God) completes all his actions whilst remaining detached from the fruits of his action.
4. A yogi is in control of his 5 senses and does not let these rule his mind.
5. There are different paths which can lead one to heaven, there is the path of knowledge (gyana yoga), the path of action (karma yoga), and the path of devotion (bhakti yoga). Another option is a combination of all of these (raja yoga).
6. When the world declines into total ignorance and people have become slaves to their physical senses, then God himself will come to impart this knowledge once again, so that the world can be saved.
The Bhagavad Gita is a beautifully written Sanksrit text, and some of the words used in the original have no English equivalent. If you are going to read a translation of the Gita, then I suggest you read one written by an experienced spiritualist and Sanskrit scholar, someone who uses the Gita on a daily basis. The depth of the words will come across better when it is coming from the heart rather than the mind.