The Upanishads are the writings from various times and sources which conclude the Indian Vedas, and are therefore known collectively as Vedanta ('end of the Vedas'). The word itself is derived from the Sanskrit for 'to sit down near': 'Upa'=near, 'ni'=down, and 'sad'=to sit. It is thought that the name comes from the fact that the Upanishads were originally secret teachings, to be imparted by teachers only to students of the highest merit and sincerity, and only individually, in close meetings, in which the student, presumably, would be sitting close at the feet of his guru.
The Upanishads are the summit of ancient Indian spiritual and philosophical writing, and are traditionally 108 in number, though of that 108 only 10 or 12 are regarded as 'classics'. Some sources also give the actual number of Upanishads in existence as over 300, with the number having been whittled down to 108 due to the mystical significance of that number in Vedic philosophy.
Like books of the Christian Bible, the Upanishads were written by various different authors at different times, with the Brihadaranyaka and the Kandogya being among the oldest. Upanishadic thought is therefore thought to have evolved over centuries, with realized sages and seers adding their commentaries and interpretations as time went by, much as in Zen literature. There are many contradictions within the Upanishads of the larger body of teachings of which they are a part, i.e. the Vedas, with some Upanishads declaring that the Vedic teachings are inferior. The Upanishads emphasize higher knowledge and realization of the Self as opposed to mere ritual and worship of religious icons.
The discovery by Western culture of the Upanishads began to gain true force when the Modernist poets, particularly Ezra Pound and T.S. Eliot, 'discovered' them and made extensive allusions and references to them in their most famous works, The Cantos (Pound) and The Waste Land (Eliot). It is not unfair to say that the Upanishads began to exert a greater and greater influence on the revolutions which have gripped Western thought in the 20th century, as can be seen in the proliferation of Hindu images, music and philosophy in the 'psychedelic' art and thought of the Sixties, which draw heavily on the cosmology and ideals of the Vedas.
The 108 principal Upanishads are listed here, grouped into the Vedas to which they belong, with the Classical (most important) Upanishads in bold: