Each Vedas are divided into four major categories (i.e., Samhita, Brahmanas, Aryanakas and Upanishads). The last part of each Vedas is called Upanishads, which means “Sitting Next to a Teacher”. This part is also known as Vedanta (i.e., the last part of the Vedas).

There are 108 Upanishads, 13 of which are the principal ones. The Upanishads contain various ideas related to the Yajnas, body, and Universe, but most of the ideas are related to Atman and Brahman.

The oldest Upanishads are in prose, but the later ones are in metres. The Brihadaranyaka Upanishad and the Chhandogya Upanishad are among the earliest Upanishads. The early period of the Upanishad belongs to the period c. 1000-500 BCE. These texts mark the first clear reference to certain key ideas and practices related to the Hindu and other roots of Indian philosophical and religious traditions.

It is considered that the Upanishads are the compilations of different authors from different parts of Northern India over the centuries.

Ref. Singh, Upinder. A History of Ancient and Early Medieval India.