Bhagavad Gita is the gist of all Upanishads instructed to Arjuna by Shri Krishna on the battle field of Kurukshetra during the Mahabharata war.
The narration is present in the Bhishma Parva of the epic Mahabharat written by Veda Vyasa. It is accepted as a smriti and scholars use it as smriti praman while deriving philosophical conclusions. The Bhagavad Gita is the most popular sacred text in Hinduism and has the same status in Hinduism as Bible has in Christianity. So the Bhagavad Gita is used as the holy book in courts for the purpose of taking oaths.
The Bhagavad Gita as a stand alone text comprises of eighteen chapters dealing with nature of soul to the paths of liberating the soul from this material world. Starting from Dharma to Karma, in only 700 verses it sheds light on almost all the important aspects of spiritualism. Hence, it is the most concise and definitive source of spiritual knowledge for a common man.