The Upanishads are perhaps some of the most beautiful pieces of religious poetry in the whole canon of eastern literature. The short verses, along with the Vedas, form the backbone of Hindu scripture. As such they also have an influence on later Indian religious movements such as Jainism and Buddhism.
The Upanishads were composed over several centuries with the oldest dated to around the seventh century BCE. They deal with meditation and the nature of divinity and ask the core questions of philosophy: Who is the Knower? What makes my mind think? Does life have a purpose? Is life governed by chance? What is the cause of the Cosmos?
The Upanishads try to solve these mysteries and seek knowledge of the ultimate reality beyond ordinary consciousness. The Upanishads also contain the first and most definitive explications of “aum” as a divine word, the cosmic vibration that underlies all existence.
Upanishad comes from the Sanskrit and literally means “sitting down near.” The title refers to the novice sitting beside the guru who instructs the novice. Instruction is intended to be private and confidential and not to be publicly spoken.
The language of the Upanishads is Sanskrit with the oldest Upanishads  composed in prose, while later Upanishads were composed in verse.
Traditionally, there were over two hundred Sanskrit Upanishads, with the philosopher Shankara credited with 11 of them. The Upanishads commented on by Shankara are generally regarded as the oldest. The most commonly cited collection, the Muktika Upanishad lists 108 Upanishads.
In 1656, at the order of the Mogul prince Dara Shikoh, the Upanishads were translated from Sanskrit into Persian in order to find monotheism in the verses and thus try and find a common link between Hinduism and the Mogul’s Islam.
The Upanishads are considered mystic reflections upon the Vedas, and are collectively known as Vedanta or “the culmination of the Vedas.” Different Upanishads are affiliated with the four major Vedas (Rig veda, Yajur veda, Sama veda and Atharva veda). The Upanishads were originally related orally by the Vedic teachers.
The Vedas are considered holy texts while the Upanishads are commentaries on them. As such the Vedas are linked to the classical and older Hinduism while the Upanishads are credited with having a great impact on the religion we know today as Hinduism.
The Vedanta, is actually composed of two sections -- the Aranyakas, which means “of the forest,” details meditative yogic practices and contemplations upon the mystic one. The second section, The Upanishad combines and sets down the mystical ideas that started in the earlier Vedic hymns. They are not regarded as a philosophy by themselves but form meditations and teachings for those advanced enough to benefit from their wisdom.
The Upanishads detail some basic Hindu beliefs, including belief in a world soul, a universal spirit, Brahman, and an individual soul, or Atman. Brahman is the ultimate, absolute infinite existence, the sum total of all that ever is, was, or ever shall be. Brahman is not a god in the monotheistic sense, as he/she/it has no limiting characteristics, not even those of being and non-being, and this is reflected in the fact that in Sanskrit, the word Brahman has no gender. Dvaita philosophy holds that Brahman is ultimately a personal God, Vishnu, or Krishna.