The Upanishads ("sitting down and listening") were one of the most important Shruti Hindu texts. The Upanishads were texts that were written and recited by Brahmins, explaining to Hindus how to live there life. There are over 200 of these texts that were written and recited. The Upanishads introduced the concepts of Karma (Do good things, good things happen), Samsara (the cycle of rebirth), Moksha (the release from Samsara), Atman (Inner-self peace), and Brahman (a state of enlightenment). From the Upanishads, the Hindus being taught realized that the only way to ever reach the state of enlightenment is through their own, personal effort. Unlike the Vedas, the Upanishads gave a reason as to why people were on Earth, rather than how to act.

http://hinduism.about.com/od/scripturesepics/a/upanishads.htm

http://www.sacred-texts.com/hin/

http://en.wikipedia.org/wiki/Upanishads

http://www.hindunet.org/upanishads/