Upanishads contains sacred treasures of Holy Texts, which were written in Sanskrit, and it also includes Vedas, Vedic rituals, fire sacrifices, festivals and ceremonies etc. Upanishads helps in imparting divine knowledge among the holy people, and it occupies as a backbone to Hinduism.
The Upanishads contains Vedanta Philosophies, which tells about the relationship between the almighty with our Soul. The concepts of finding the god, within ourselves are clearly mentioned in the Upanishads.
The holy Bhagavad Gita, Puranas and the Vedas are considered as a part of Upanishads.Upanishads have been dedicated to the famous sages like Yajnavalkya, Atreya, Narada, Vyasa, and Sanatkumaras. Ancient chaste and holy women saints like Maitreyi and Gargi are also mentioned in the Upanishads, and they have also contributed certain portions of texts in the Upanishads.
In the Upanishads, the greatness of the divine deities such as Varuna, Yama, Vayu, Surya and Chandra are also mentioned.The Upanishads are related with the four Vedas: Rig, Sama, Yajur and Atharva.
Madhwa saints like Vyasaraja, Sripadaraja and Madhwacharya have added their philosophies, Brahmasutras, Mantras and teachings into the Upanishads, and made it into very sweet. Upanishads throws the light in the minds of the readers, and it gives great peace of mind. It is very essential to learn the contents mentioned in the ancient Upanishads, in order to live our life as per the teachings mentioned in that.
The 13th century saint-poet Sri Dnyaneshwar, once made an ox to utter the holy Upanishads, when he was insulted by the learned scholars. After knowing about his great spiritual powers, the scholars bowed their heads before him, and gave him a lot of respect. Likewise lot of saints have added beauty and sweetness to the ancient Upanishads and made it into pious, holy and divine text.