Upanishad means the inner or mystic teaching. The term Upanishad is derived from upa (near), ni (down) and s(h)ad (to sit), i.e., sitting down near. Groups of pupils sit near the teacher to learn from him the secret doctrine. In the quietude of the forest hermitages the Upanishad thinkers pondered on the problems of deepest concerns and communicated their knowledge to fit pupils near them. Samkara derives the word Upanishad as a substitute from the root sad, 'to loosen.,' 'to reach' or 'to destroy' with Upa and ni as prefixes and kvip as termination. If this determination is accepted, upanishad means brahma-knowledge by which ignorance is loosened or destroyed. The treatises that deal with brahma-knowledge are called the Upanishads and so pass for the Vedanta. The different derivations together make out that the Upanishads give us both spiritual vision and philosophical argument. There is a core of certainty which is essentially incommunicable except by a way of life. It is by a strictly personal effort that one can reach the truth.
The Upanishads more clearly set forth the prime Vedic doctrines like Self-realization, yoga and meditation, karma and reincarnation, which were hidden or kept veiled under the symbols of the older mystery religion. The older Upanishads are usually affixed to a particularly Veda, through a Brahmana or Aranyaka. The more recent ones are not. The Upanishads became prevalent some centuries before the time of Krishna and Buddha.
The main figure in the Upanishads, though not present in many of them, is the sage Yajnavalkya. Most of the great teachings of later Hindu and Buddhist philosophy derive from him. He taught the great doctrine of "neti-neti", the view that truth can be found only through the negation of all thoughts about it. Other important Upanishadic sages are Uddalaka Aruni, Shwetaketu, Shandilya, Aitareya, Pippalada, Sanat Kumara. Many earlier Vedic teachers like Manu, Brihaspati, Ayasya and Narada are also found in the Upanishads.
In the Upanishads the spiritual meanings of the Vedic texts are brought out and emphasized in their own right.
The Upanishads have been perennial sources of spiritual knowledge. The word upanishhad means secret and sacred knowledge. This word occurs in the Upanishads themselves in more than a dozen places in this sense. The word also means "Texts incorporating such knowledge." There are ten principal Upanishads. Other than these, a few more like Shvetaashwatara and KaushiTaki are also considered important. Though it is known that even before Sri Shankara, commentaries were written on the Upanishads, these have been lost. Sri Shankara's commentaries on the principal Upanishads are the earliest available. Sri Ramanuja has not written any commentaries on them, but a later disciple Sri Rangaramanuja has written them. Sri Madhvacharya has written commentaries (bhaashya-s) on the ten principal Upanishads. Interpretation of passages from these and other Upanishads is also discussed by him in his Suutra-Bhaashya, which is mainly about the interpretation of Shruti texts and also in his other major works like Anu-vyaakhyaana, Vishnu-tatva-vinirNaya, and Tattvodyota.