upa (near) - ni (down) - shad (to sit); so lit. "to sit down near [a guru]"--close so as not to divulge the secret doctrine. Over 200 of them written between 800 B.C.E. to 200 B.C.E.
Traditionally, there are 108 Upanishads (major), which are as follows:
Aitareya and the Kauhsitaki which belong to Rig Veda
Chandogya and Kena to Samaveda
Taittiriya, Katha, Shvetashvatara, Brihadaranyaka and Isha to Yajur Veda
Prashna, Mundaka and Mandukya to Atharvaveda.
Five Upanishadic Themes
Saguna Brahman (sa=with + guna=quality). A form of Brahman with qualities. Connected to personal theism in the Hindu tradition.
Nirguna Brahman (nir=no) Brahman without qualities, i.e., an undifferentited unity. This is the Brahman stressed by Advaita Vedanta.
Tat twam asi =that thou art. Thou art Brahman, because Brahman=Atman? Please read about Herman's various interpretations of this phrase (Herman , pp. 116-124). Questions to ask are which Brahman is referred to? Which self? self, Self, jiva (sinful, sensible soul)? And which "art"? Will it be the "is" of existence? E.g. you exist, I exist, Brahman exists? Or the "is" of predication? The sun is red, or Christ is divine? Or, finally, the "is" of identity. God is God, Brahman is Atman (because Brahman and Atman are identical). Which view do you think Advaita Vedanta takes? Which "is" is it if we say that Atman is Brahman in us, but Atman is not identical with Brahman?
WAYS OF SPIRITUAL LIBERATION
Jnana Yoga - The Yoga of Knowledge (jnana) very strong in Upanishads. The yoga of the philosopher.
Karma Yoga - The Yoga of Action (deeds, works)
Bhakti Yoga - The Yoga of Devotion--personal theism and doctrine of grace. Faith and Doctrine of Election, i.e., a personal God chooses you for salvation. Strong in Bhagavad-gita and parts of the Upanishads.
Dhyana Yoga - The Yoga of Meditation. Dhyana was translated into the Chinese Ch'an and the Japanese Zen.
The Yoga of Renunciation - The Yoga of the ascetic and the devotee. Could possibly call this "Tapas" yoga, a yoga with austerities but not necessarily meditation.
Tantric Yoga - The Way of Experience. A later movement in Hinduism and Buddhism. Called a short-cut way. Involves a total immersion in all experience, doing all that is prohibited while still remaining immune and pure, and "coming out the other side" totally liberated.
One of the main teachings of the Upanishads is pantheism. Brahman is everywhere and is in everything. A more accurate designation could be "panentheism."