This is the introduction to the Upanishads, as explained by Sanderson Beck in his book, 'The Wisdom Bible'. I thought it might help clarify the subject (goodness knows I had trouble deciphering it until I found this book).

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"Introduction to the Upanishads"

The Upanishads were written by sages of India between the eighth and fourth centuries BC. They are the final part of the Vedas and the basis for the philosophy of Vedanta, which means the end of the Vedas. The Vedas are the most ancient and sacred scripture of India; the name signifies wisdom. The Rig Veda gives the verses or hymns. The Yajur Veda has sacrificial formulas. The Sama Veda contains the melodies of the chants. The Atharva Veda is magic formulas. The four parts of the Vedas are the following: Samhita is the main part and has hymns, prayers, and litanies; the Brahmanas discuss the meaning of the sacrificial rites and ceremonies; the Aranyakas are forest-texts; and the Upanishads are the teachings of the sages on mystical experience. As part of the Vedas the Upanishads are considered revealed scripture.

The word Upanishad literally means "sitting down near" and implies studying with a spiritual teacher. The seven Upanishads presented complete in this collection are drawn from the twelve principal Upanishads and appear in what is considered their chronological order, the KENA, KATHA, and ISHA being considered pre-Buddhistic and thus from the eighth or seventh centuries BC. The name KENA comes from the first word which means "By whom." ISHA comes from the first word meaning "Lord." PRASHNA comes from the word for "question."

The gods referred to in these Upanishads are Agni the god of fire, Vayu the god of air or wind, Indra the god of heroic power and storms, Rudra a god of destruction and of healing, Savitri a sun god or goddess, Brahma the creator, and Vishnu the preserver. In the KATHA UPANISHAD 5 the city of eleven gates refers to the nine openings in the body, the navel, and the sagittal suture on top of the head, and in the SHVETASHVATARA UPANISHAD 3, the nine-gated city refers merely to the body's nine openings. The triad in MUNDAKA 1 refers to the first three Vedas, while the triad in SHVETASHVATARA 1 seems to refer to three aspects of God. In MUNDAKA 3 Vedanta means the end of the Vedas.

© Sanderson Beck, 1996- The Wisdom Bible