Excellent rendition

http://en.wikipedia.org/wiki/Upanishads
The key phrase of the Upanishads, to Advaita Vedanta, is तत् त्वं असि "Tat Tvam Asi" (That thou art). Vedantins believe that in the end, the ultimate, formless, inconceivable Brahman is the same as our soul, Atman. We only have to realize this through discrimination. (However, interpretations of this phrase differ.)[13] Verses 6, 7 & 8 of Isha Upanishad:
......

Whoever sees all beings in the soul and the soul in all beings...
What delusion or sorrow is there for one who sees unity?
It has filled all. It is radiant, incorporeal, invulnerable...
Wise, intelligent, encompassing, self-existent,
It organizes objects throughout eternity.

.....
The Upanishads also contain the first and most definitive explications of the divine syllable Aum or OM, the cosmic vibration that underlies all existence. The mantra "Aum Shanti Shanti Shanti" (the soundless sound, peace, peace, peace)is often found in the Upanishads. 'Devotion to God' (Sanskrit: bhakti) is foreshadowed in Upanishadic literature, and was later realized by texts such as the Bhagavad Gita.[14]