Bhagavad-Gita is a unique classic, unique for many Sons, and for many purposes. It is essentially a philosophical work and less religious in content and hence universally appealing and aspiring.

Broadly accepted date of Gita is about 10th century BC or 950 BC. Earliest dating of gita is around 3000 BC to 2500 BC. Based on the popularly accepted date Bhagavad-Gita is at least 3000 years old. Being the oldest scripture with the universal circulation still continuing, naturally it might have gone through many interpolations, additions or omissions.

Gita is basically a part and parcel of Mahabharata, (book of more than one lakh slokas or verses). However it is considered to be a Upanishad by itself, nay the essence of all the Upanishads and vedanta and next only to Vedas(according to traditional Hindus).

Gita is more philosophical and less religious work, unlike the Bible and Quran. Perhaps only other genre of the Gita kind are the Buddhist texts which are also more philosophical and less religious.

As has been stated, Gita is the part of Bhishma Parva in Mahabharata. Basically it is a dialogue between the lord Krishna (the God in Hindu view) and Arjuna, a pandava warrior, on *Kurukshetra' battle field, which is the mother of all battles ever fought in human history.