The Upanishads mostly explain in details Vedic Theology including metaphysics, spiritual and mystical powers and concepts of the God. The literal meaning of Upanishad itself is "the knowledge of realising and visualising God". In some Upanishads, some of the Vedic hymns are produced as such. These texts mostly deal with the concepts, charateristics and manifestations of powers of the God, nature and properties of the soul, its relationship with the God; sometimes in figurative manner or in symbolic stories. in these books, most complex philosophical concepts and spiritual experiences have been presented through lucid dialogues. Upanishad is also called Shruti.
References in mythological literature indicates the existence of 1000 Upanishads in ancient times. It is said that in ancient times each Shaakha of a Veda had its own Upanishad. But like Shakaas, upanishads are also untraceable now. The major 11 ones are as noted below (with the Veda they belong to):-
Isha (Yajur):- Ishopanishad provides the basic tenats of the Vedas in brief. Important Vedic Concepts relating to the God, soul and nature, moral duties, monotheism, life after death, and other complex philosophical concepts are clearly dealt therein. Except one mantra, the entire Ishopanishad is the last chapter of YajurVed and is the gist of Vedic Religion.