Hindu religion is a collection of diverse and varied philosophies, based on diverse and varied texts. Still, there are some common threads shared between most types of Hindu religion, based on some common scriptures.
The Vedas (Shrutis)
The Vedas are the central and most ancient texts of Hindu religion. There are four Vedas-Rig Veda, Sama Veda, Yajur Veda and Atharva Veda-and together they make up a very large collection of writings.
Hindus regard the Vedas as not having been created or invented by anyone, but instead as the repositories of eternal wisdom. The Vedas are thus called the shrutis (texts and concepts that are said to be revealed or discovered rather than invented) rather than smritis, which are written by individual humans.
The Upanishads (Shrutis)
The Upanishads are philosophical and spiritual analyses and meditations on the Vedas; as such, they are often referred to as Vedanta (the end or goal of the Vedas). The Upanishads are in the format of conversations and debates between sages, teachers and students, and they propound the philosophy of Advaita, which means non-duality. Advaita philosophy is non-dualistic: it accepts no creator that is apart from the physical universe, and accepts no division between the material and the "spiritual". There are many Upanishads, eleven of which are famous: Katha, Isha, Kena, Mundaka, Shvetasvatara, Prashna, Mandukya, Aitareya, Taittiriya, Brihadaranyaka and Chandogya.
Smritis
Literally "that which is remembered", smritis are distinguished from shrutis in that they have an author or creator whose name is remembered. They are secondary in authority to the shrutis and are wildly diverse, often contradicting each other. They include a vast collection of works including the dharmashastras (law books), the itihasas (literally "histories", referring to the great Indian epics, which will be discussed in greater detail) and the puranas (a collection of tales). The smritis also include the Bhagavad-Gita, the most widely known Hindu text, which is considered to be a distillation of the wisdom in the Vedas and Upanishads.
Itihasas
These are the great Indian epics, the most widely known of which are the Ramayana and the Mahabharata. Both of these are family sagas, the stories of warfare and political intrigue and dynastic struggles. The Mahabharata includes the Bhagavad-Gita, which consists of the teachings of Lord Krishna (considered an incarnation of the god Vishnu) to Arjuna (a character in the epic). Itihasas fall under the category of smritis.