The Upanishads: Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Taittiriya, Aitareya, Shvetashvatara, Chandogya, Brihadaranyaka

The word Upanishad is derived from the Sanskrit verb 'sad,' which means to sit, and two prepositions, 'upa' (under) and 'ni' (at). The sense is thus of sitting for instruction at the feet of a teacher.
The Upanishads are sacred treatises, spiritual and philosophical in nature. Vedic literature is divided into two broad groups. The first is known as karma kanda, the section that deals with rituals and rites. This has the samhitas, that is, compilations of mantras or hymns. The four samhitas are the Rig Veda, the Sama Veda, the Yajur Veda and the Atharva Veda The mantras require commentaries as explanations. These are the brahmanas. The samhitas and the brahmanas are together known as karma kanda.
The second section of Vedic literature is Jnana kanda, the part that deals with knowledge. This segment is the spiritual and philosophical part and consists of the aranyakas and the Upanishads.
The earliest Upanishads were probably composed between the years 800 B.C. and 400 B.C. The major Upanishads are ten or eleven in number. This is based on the fact that Shankaracharya (800 A.D.) wrote his celebrated commentaries of ten of the Upanishads.
The eleven major Upanishads listed are fairly short. It is of course possible that the text that is now available does not constitute the entirety of the original Upanishad. This, for example, seems to have happened with the Mandukya Upanishad Two Upanishads are however fairly long. These are the Chandogya and the Brihadaranyaka.
It is now impossible to establish the chronological order in which the Upanishads were composed. But the most likely order seems to be Brihadaranyaka, Chandogya, Taittiriya, Kena, Katha, Isha, Mundaka, Prashna, Mandukya, Shvetashvatara and Aitareya.
The teachings of the Upanishads, together with that of the Bhagavad Gita, form the basis of Vedanta philosophy.