What the Sāma Veda is
The Sāma Veda's distinctive character comes not from its words but from what is done with them: nearly every verse also appears in the Ṛg Veda, but here each is provided with musical notation and reorganized for chanting by the Udgātṛ priest during the Soma sacrifice. Where the Ṛg Veda's Hotṛ priest recites, the Sāma Veda's Udgātṛ sings — the same words transformed by melody, rhythm, and ornamentation into a distinct ritual performance.
The text is organized into two main sections: the Pūrvārcika ("former collection"), containing verses arranged by deity (Agni, Indra, Soma Pavamāna) for straightforward chanting, and the Uttarārcika ("latter collection"), containing verses arranged by the specific ritual sequences in which they are sung. Several recensions (śākhās) once existed; the Kauthuma recension is the most widely preserved and studied today, alongside the Rāṇāyanīya and Jaiminīya recensions.
Mantras from the Sāma Veda tradition, in full
The Sāma Veda's own verses are, with rare exception, drawn from the Ṛg Veda — including the Gāyatrī mantra itself, which the Sāma Veda tradition also chants (in its own melodic form) during ritual. Its most significant original contribution to this codex's Mantras section comes indirectly, through the Chāndogya Upaniṣad, which belongs to this Veda:
The Chāndogya Upaniṣad
The Sāma Veda's most significant contribution to the tradition's philosophical literature is the Chāndogya Upaniṣad, one of the oldest and most important of the major Upaniṣads — the source of the famous tat tvam asi ("that thou art") teaching, and of an extended meditation on the syllable Oṃ as the essence of the Sāma chant itself. The Upaniṣad opens by treating Oṃ (there called udgītha) as the very essence toward which the entire practice of sāman chanting points — connecting this Veda's musical character directly to its philosophical culmination.
Composition and compilation
Since the Sāma Veda largely reuses Ṛg Vedic verses, its own distinctive contribution is the layer of musical notation and ritual reorganization, generally dated somewhat later than the core Ṛg Vedic hymns it draws from, in the same broad Vedic period.
Michael Witzel, "Vedas and Upaniṣads," in The Blackwell Companion to Hinduism (ed. Flood, Blackwell, 2003). Patrick Olivelle, The Early Upaniṣads (Oxford University Press, 1998) — for the Chāndogya Upaniṣad.