Upanishad means the spiritual way of thinking developed by the Indian scholars. These Upanishads are known as "वेदान्त" as they are included in the final phase of the Vedic literature. Vedant means the end of the knowledge. After obtaining this much knowledge, nothing remains pending. It is clearly mentioned in the Upanishad that we obtain all sorts knowledge merely by realizing the supreme being.
According to Aadi Shankaracharya, "Upanishad means the learning which helps to realize the supreme being, removes affection and makes us free from the worldly bondages."
There are more than 250 books known as Upanishads. According to the Indian tradition, there are 108 Upanishads. But all of them do not belong to the ancient time. Some of them belong to the modern times. The main Upanishads of the ancient times include Ish, Ken, Kath, Prashna, Mundak, Mandukya, Tittari, Aitreya, Chhandogya and Bruhadaranyaka. Apart from this, Shwetashwatar, Kaushitaki and Maitrayani are also regarded as the Upanishad of the ancient times. Aadi Shankaracharya and other preceptors accepted the importance of these Upanishads by writing exposition upon them.
The supreme being has been mentioned as "Brahm" in the Upanishads. The description of both forms of the supreme being as manifest form and unmanifest form is found in the Upanishad. It contains different ways to attain the supreme being. The condition of the soul after death has been explained in Kathopanishad. We also find the minute interpretation of the living creature, the universe and the god in Upanishad. Mainly the doctrine of monism has been presented in Upanishad. In Mundakopanishad, the living creature and the supreme being are compared with beautiful birds and are shown sitting on a tree in the form of the world. Monism has been explained by establishing uniformity between both of them.
The style of Upanishad is simple and interactive which contains the discussion about serious issues in the form of questions and answers. The language is easy and according to the subject matter.
The rich culture of India has given a divine gift to the mankind in the form of Upanishad.