As hindu-blog points out, the Upanisads are based on a longstanding oral tradition of uncertain duration, making precise dating extremely difficult:
The Upanishads and Vedas were rendered orally and were passed on for generations before being written. Nobody is sure about the actual dates of these texts.¹
But there is some debate here, and some scholars say these scriptures became formally known as the Upanisads around 800 CE.
The Upanisads are premised on the idea that a sacred teacher (guru) imparts esoteric mystical knowledge to those ready to learn. This type of learning is said to be mostly experiential instead of conceptual.
The texts contain several key Hindu images. One of the most accessible, found in the Mundaka-Upanisad, is that of two birds sitting on a tree. One bird eats the sweet fruit on the branches while the other watches. The eater symbolizes the active, temporal and perishable aspects of creation, while the watcher symbolizes an immovable, omniscient and eternal Self.
In another important Upanisad, the Katha-Upanisad, a young man, Nachiketa, seeks the wisdom of immortality by entering into dialogue with his teacher Yama (death). Yama initially advises Nachiketa to pursue anything else but this particular question because of its inherent difficulties. But the young man persists and, after recognizing his sincerity and determination to achieve ultimate truth, Yama begins to instruct Nachiketa on the nature of the eternal self, as he understands it.
‘Know the Self to be sitting in the chariot, the body to be the chariot, the intellect (buddhi) the charioteer, and the mind the reins.’
‘The senses they call the horses, the objects of the senses their roads. When he (the Highest Self) is in union with the body, the senses, and the mind, then wise people call him the Enjoyer.’
‘He who has no understanding and whose mind (the reins) is never firmly held, his senses (horses) are unmanageable, like vicious horses of a charioteer.’
‘But he who has understanding and whose mind is always firmly held, his senses are under control, like good horses of a charioteer.’
‘He who has no understanding, who is unmindful and always impure, never reaches that place, but enters into the round of births.’
‘But he who has understanding, who is mindful and always pure, reaches indeed that place, from whence he is not born again.’²
Altogether there are over 200 Upanisads, but not all are seen as equally important.