The Upanishads (Sanskrit: उपनिषद्, IAST: Upaniṣad, IPA: [upəniʂəd]) are a collection of philosophical texts which form the theoretical basis for the Hindu religion. They are also known as Vedanta (“the end of the Veda“). The Upanishads are considered by orthodox Hindus to contain revealed truths (Sruti) concerning the nature of ultimate reality (brahman) and describing the character and form of human salvation (moksha). The Upanishads are found mostly in the concluding part of the Brahmanas and Aranyakas and have been passed down in oral tradition.
More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main (mukhya) Upanishads. With the Bhagavad Gita and the Brahmasutra (known collectively as the Prasthanatrayi), the mukhya Upanishads provide a foundation for the several later schools of Vedanta, among them, two influential monistic schools of Hinduism Themukhya Upanishads all predate the Common Era, possibly from the Pre-Buddhist period (6th century BCE) down to the Maurya period. The remainder of the Muktika canon was mostly composed during medieval Hinduism, but new Upanishads continued being composed in the early modern and modern era, down to at least the 19th century.
Excerpt from Wikipedia Article: Upanishads