Hinduism has a number of sacred books that are collectively part of their corpus of sacred literature. The earliest and most ancient and in some ways I guess the widest held to be given authority is a group of four collections of works known collectively as the Samhitas, but are individually known as the Four Vedas: The Rigveda, the Samaveda, the Yarjurveda and the Atharvaveda.
These four works represent some of the earliest material in Hinduism. Now what are the Rigvedas, the Samavedas, the Yarjurvedas and so forth? Well, essentially this is a corpus of material that is in praise to certain early gods and godesses that were worshipped in the very earliest days of what we now call the Vedic period in early, early proto-Hinduism. The Rigveda, for example, is actually divided into 10 smaller books and within these 10 books collectively it contains 1028 metrical hymns that are sung to these various gods. Most of these gods are associated with nature such as god of fire, god of sky, god of sun and wind and the dawn and so forth. These early hymns of praise to god later get reworked into chants, where people would chant these and they believe that by chanting these hymns in another metrical form it releases certain kinds of spiritual power and that is known as the Samaveda, and this happens throughout the Vedic documents. So that’s a very important understanding to understand how the early development occurs.
Later, early proto-Hindu writers began to add to the Vedas different appendices, mainly commentaries on the Vedas or other kinds of speculative, philosophical treatises; so these get added to the Samhitas until eventually you have a group of writings known as the Brahmanas, which are basically commentaries; and then you have the Aranyakas, which are treatises, and then finally, and most importantly, the Upanishads. The Upanishads will take some of our time to fully explain because the Upanishads represents the most important body of literature in the Hindu corpus. The Upanishads is the basis of what we today call Hindu philosophy, it makes a huge part of various reflections of Hinduism that appear throughout the ages; and so the Upanishads represents a very significant document. I think it’s one of the most important documents to be aware of if you want to study Hinduism. The Upanishads along with the Rigveda are widely believed to be two of the most sacred documents in the Hindu religion.