The Upanishads are an integral part of ancient Indian scriptures. In simple words, Upanishads are the comments made on the Vedas that comes in the end of each Vedas. For this reason it is also referred to as Vedanta, means the end of the Veda. In that era, the Guru would sit on a raised top near a tree bark and the disciple near him on the ground. The word Upanishad also means to sit down close to… on a whole we can say it means to sit down close to the Guru. And the Guru can be anyone, who is teaching us something constructive.
The exact period of the origin of Upanishad is not clear because it was developed over a long time span. However, it can be confirmed that the Upanishads existed during the early civilization of the Indus valley.
Upanishads are of various kinds and are related to one of the Vedas; like Aitareya Upanishad is linked to Rig-Veda, Brihadaranyaka, Isha to Yajur Veda, Taittiriya to Yajur Veda, Chandogya and Kena to Sama Veda, Mundaka and Mandukya to Atharva Veda, Katha, Prashna, Shvetashvatara to Yajur Veda. There are about 108 Upanishads of which the above ones are the main Upanishads.
As such the Upanishads are the sum up of Vedas and hence it is the base of the teachings of Indian scriptures. A lot of Indian theories that are now globally accepted at a spiritual level are found in Upanishads. In the Upanishads, there are references to many spiritual concepts such as the law of Karma, Yoga, life after death and lots more.