Upanishads (उपनिषद्‌) are part of the Hindu Shruti scriptures which primarily discuss meditation and philosophy and are seen as religious instructions by most schools of Hinduism. They also contain transcripts of various spiritual debates or discussions, and of the 123 books considered to be part of the Upanishads, 12 are accepted by all Hindus as primary. The term Upanishad derives from the Sanskrit words upa (near), ni (down) and s(h)ad (to sit) i.e., sitting down near; implying the act of listening to a spiritual teacher. The Upanishads are commentaries on the Vedas, their putative end and essence, and thus known as Vedanta ("End of the Veda").

The Major Upanishads

Different Upanishads serve as commentaries or extensions of each of the four Vedas (Rig Veda, Yajur Veda, Sama Veda and Atharva Veda). The oldest and longest of the Upanishads are the Brihad-Aranyaka and the Chandogya; scholars vary on when they first were written and estimates range the 16th to 7th century BCE. There is great scholarly consensus that many of the early Upanishads are pre-Buddhistic. Initially there were over two hundred Upanishads but only fifteen or so were considered to be primary by the philosopher Shankara. The Upanishads were not fully recorded until 1656, at the order of Dara Shakoh.

These philosophical and meditative tracts form the backbone of Hindu thought. Of the early Upanishads, the Aiterya and Kaushitaki belong to the Rig Veda, Kena and Chandogya to the Sama, Isha and Taittereya and Brihadaranyaka to the Yajur, and Prasna and Mundaka to the Atharva. (associated Upanishad and Vedic book taken from Radhakrishnan Indian Philosophy, Vol.1). In addition, the Mandukya, Katha, Svetashvatara are very important. Others also include Mahanarayana and Maitri Upanishads as key.

Whence the Upanishads?

Often, critics of the Hindu/Vedic tradition will use the term Brahminical to imply a karma-kanda, or ritual-based mode of worship, sort of stuck in the four Vedic books and Brahamanas (Vedic ritual books). However, it is widely acknowledged that those who wrote the mystic verse of the Upanishads were in all likelihood Brahmins as well.

Scholarly breakdowns of the Vedic books see the four Vedas as poetic liturgy, collectively called mantras or samhitas, adoration and supplication to a sort of melded monist and henotheist notion of the Gods/Goddesses and an overarching Order (Rta) that transcended even the Gods and stemmed from One Ultimate Source. The Brahmanas were a collection of ritual instructions, books detailing the priestly functions (which first were available to all men, and so concretized into strictly Brahmin privilege). These came after the Mantras.

Then we have the Upanishads, which consist of the Aranyakas and Upanishads. Aranyaka means forest, and these most probably grew as a sort of subtle rejection of the Brahmanas: they detail meditative yogic practices, contemplations of the mystic one and the manifold manifested principles. The Upanishads, finally, meaning 'Sitting Near' (implied are the Guru's sacred feet), culminate. The Upanishads basically realized all the monist and universal mystical ideas that saw their nascence in earlier Vedic hymns, and have exerted an influence unprecedented on the rest of Hindu and Indian philosophy. However, by adherents they are not considered philosophy alone, and form meditations and practical teachings for those advanced enough to benefit from their wisdom.

What do the Upanishads contain?

The Taittiriya Upanishad says this in the Ninth Chapter:

"He who knows the Bliss of Brahman, whence words together with the mind turn away, unable to reach It? He is not afraid of anything whatsoever. He does not distress himself with the thought Why did I not do what is good? Why did I do what is evil? Whosoever knows this regards both these as Atman; indeed he cherishes both these as Atman. Such, indeed, is the Upanishad, the secret knowledge of Brahman."

The Philosophy of the Upanishads

Due to their mystic nature and intense philosophical bent that does away with all ritual and completely embraces principals of One Brahman and the inner Atman, the Upanishads have a universal feel that has led to their explication in numerous manners, giving birth to the three schools of Vedanta.

To sum up all the Upanishads in one phrase would be "Tat Twam Asi" (Thou Art That). In the end, the ultimate, formless, inconceivable Brahman is the same as our soul, Atman. We only have to realize it through discrimination and piercing through Maya. A distinctive quotation that is indicative of the call to self-realization, one that inspired Somerset Maugham in titling a book he wrote on Christopher Isherwood, is as follows:

" Get up! Wake up! Seek the guidance of an

Illumined teacher and realize the Self.

Sharp like a razor's edge is the path,

The sages say, difficult to traverse."

--- Death Instructing Nachiketa in the Katha (Word) Upanishad

The Upanishads also contain the first and most definitive explications of ॐ 'Aum', as the divine word, the cosmic vibration that underlies all existence and contains multiple trinities of being and principles subsumed into its One Self.

The Isha says of the Self:

"Whoever sees all beings in the soul

and the soul in all beings

does not shrink away from this.

In whom all beings have become one with the knowing soul

what delusion or sorrow is there for the one who sees unity?

It has filled all.

It is radiant, incorporeal, invulnerable,

without tendons, pure, untouched by evil.

Wise, intelligent, encompassing, self-existent,

it organizes objects throughout eternity."

"Aum Shanti Shanti Shantihi" This, too, is found first in the Upanishads, the call for tranquility, for divine stillness, for Peace everlasting.

List of Upanishads

iisha = shukla yajurveda, mukhya upanishhad
kena =  saama veda, mukhya upanishhad

kaTha = kR^ishhNa yajurveda, mukhya upanishhad

prashna = atharva veda, mukhya upanishhad
muNDaka = atharva veda, mukhya upanishhad
maaNDukya = atharva veda, mukhya upanishhad

taittiriiya = kR^ishhNa yajurveda, mukhya upanishhad 

aitareya = R^ig veda, mukhya upanishhad 

chhaandogya =  saama veda, mukhya upanishhad
bR^ihadaaraNyaka (10) = shukla yajurveda, mukhya upanishhad
brahma = kR^ishhNa yajurveda, sa.nnyaasa upanishhad
kaivalya = kR^ishhNa yajurveda, shaiva upanishhad
jaabaala(yajurveda) = shukla yajurveda, sa.nnyaasa upanishhad
shvetaashvatara = kR^ishhNa yajurveda, saamaanya upanishhad
ha.nsa = shukla yajurveda, yoga upanishhad
aaruNeya = saama veda, sa.nnyaasa upanishhad
garbha = kR^ishhNa yajurveda, saamaanya upanishhad
naaraayaNa = kR^ishhNa yajurveda, vaishhNava upanishhad
paramahaMsa = shukla yajurveda, sa.nnyaasa upanishhad
amR^ita-bindu (20) = kR^ishhNa yajurveda, yoga upanishhad
amR^ita-naada = kR^ishhNa yajurveda, yoga upanishhad
atharva-shira = atharva veda, shaiva upanishhad
atharva-shikha = atharva veda, shaiva upanishhad

maitraayaNi = saama veda, saamaanya upanishhad

kaushhiitaaki = R^ig veda, saamaanya upanishhad
bR^ihajjaabaala = atharva veda, shaiva upanishhad 
nR^isiMhataapanii =  atharva veda, vaishhNava upanishhad
kaalaagnirudra = kR^ishhNa yajurveda, shaiva upanishhad
maitreyi = saama veda, sa.nnyaasa upanishhad
subaala (30) = shukla yajurveda, saamaanya upanishhad
kshurika = kR^ishhNa yajurveda, yoga upanishhad
maantrika = shukla yajurveda, saamaanya upanishhad
sarva-saara = kR^ishhNa yajurveda, saamaanya upanishhad
niraalamba = shukla yajurveda, saamaanya upanishhad
shuka-rahasya = kR^ishhNa yajurveda, saamaanya upanishhad
vajra-suuchi = saama veda, saamaanya upanishhad
tejo-bindu = kR^ishhNa yajurveda, sa.nnyaasa upanishhad
naada-bindu = R^ig veda, yoga upanishhad
dhyaanabindu = kR^ishhNa yajurveda, yoga upanishhad
brahmavidyaa (40) = kR^ishhNa yajurveda, yoga upanishhad
yogatattva = kR^ishhNa yajurveda, yoga upanishhad
aatmabodha = R^ig veda, saamaanya upanishhad
parivraat (naaradaparivraajaka) = atharva veda, sa.nnyaasa upanishhad
tri-shhikhi = shukla yajurveda, yoga upanishhad 
siitaa = atharva veda, shaakta upanishhad
yogachuuDaamaNi = saama veda, yoga upanishhad
nirvaaNa = R^ig veda, sa.nnyaasa upanishhad
maNDalabraahmaNa = shukla yajurveda, yoga upanishhad
dakshiNaamuurti = kR^ishhNa yajurveda, shaiva upanishhad
sharabha (50) = atharva veda, shaiva upanishhad 
skanda (tripaaDvibhuuTi) = kR^ishhNa yajurveda, saamaanya upanishhad 
mahaanaaraayaNa = atharva veda, vaishhNava upanishhad
advayataaraka = shukla yajurveda, sa.nnyaasa upanishhad
raamarahasya = atharva veda, vaishhNava upanishhad
raamataapaNi = atharva veda, vaishhNava upanishhad
vaasudeva = saama veda, vaishhNava upanishhad
mudgala = R^ig veda, saamaanya upanishhad
shaaNDilya = atharva veda, yoga upanishhad
pai.ngala = shukla yajurveda, saamaanya upanishhad
bhikshuka (60) = shukla yajurveda, sa.nnyaasa upanishhad
mahat = saama veda, saamaanya upanishhad
shaariiraka = kR^ishhNa yajurveda, saamaanya upanishhad
yogashikhaa = kR^ishhNa yajurveda, yoga upanishhad
turiiyaatiita = shukla yajurveda, sa.nnyaasa upanishhad
sa.nnyaasa = saama veda, sa.nnyaasa upanishhad
paramahaMsa-parivraajaka = atharva veda, sa.nnyaasa upanishhad
akshamaalika = R^ig veda, shaiva upanishhad
avyakta = saama veda, vaishhNava upanishhad
ekaakshara = kR^ishhNa yajurveda, saamaanya upanishhad
annapuurNa (70) = atharva veda, shaakta upanishhad
suurya = atharva veda, saamaanya upanishhad
akshi = kR^ishhNa yajurveda, saamaanya upanishhad
adhyaatmaa = shukla yajurveda, saamaanya upanishhad 
kuNDika = saama veda, sa.nnyaasa upanishhad
saavitri = saama veda, saamaanya upanishhad
aatmaa = atharva veda, saamaanya upanishhad
paashupata = atharva veda, yoga upanishhad
parabrahma = atharva veda, sa.nnyaasa upanishhad
avadhuuta = kR^ishhNa yajurveda, sa.nnyaasa upanishhad
tripuraatapani (80) =  atharva veda, shaakta upanishhad
devi = atharva veda, shaakta upanishhad
tripura =  R^ig veda, shaakta upanishhad
kaTharudra = kR^ishhNa yajurveda, sa.nnyaasa upanishhad
bhaavana =  atharva veda, shaakta upanishhad
rudra-hR^idaya = kR^ishhNa yajurveda, shaiva upanishhad
yoga-kuNDalini = kR^ishhNa yajurveda, yoga upanishhad
bhasma =  atharva veda, shaiva upanishhad
rudraaksha = saama veda, shaiva upanishhad
gaNapati =  atharva veda, shaiva upanishhad
darshana (90) = saama veda, yoga upanishhad
taarasaara = shukla yajurveda, vaishhNava upanishhad
mahaavaakya = atharva veda, yoga upanishhad
paJNcha-brahma = kR^ishhNa yajurveda, shaiva upanishhad
praaNaagni-hotra = kR^ishhNa yajurveda, saamaanya upanishhad
gopaala-tapaNi = atharva veda, vaishhNava upanishhad
kR^ishhNa = atharva veda, vaishhNava upanishhad
yaaGYavalkya = shukla yajurveda, sa.nnyaasa upanishhad
varaaha = kR^ishhNa yajurveda, sa.nnyaasa upanishhad
shaatyaayani = shukla yajurveda, sa.nnyaasa upanishhad
hayagriiva (100) = atharva veda, vaishhNava upanishhad
dattaatreya = atharva veda, vaishhNava upanishhad
gaaruDa = atharva veda, vaishhNava upanishhad
kali-saNTaaraNa = kR^ishhNa yajurveda, vaishhNava upanishhad
jaabaala(saamaveda) = saama veda, shaiva upanishhad
saubhaagya = R^ig veda, shaakta upanishhad
sarasvatii-rahasya = kR^ishhNa yajurveda, shaakta upanishhad
bahvR^icha = R^ig veda, shaakta upanishhad
muktika (108) = shukla yajurveda, saamaanya upanishhad 

19 upanishhads are from Shukla Yajur veda and have the Shaanti beginning 'puurNamada.'

32 upanishhads are from krishna yajurveda and have the Shaanti beginning 'sahanaavavatu.'

16 upanishhads are from Saama veda and have the Shaanti beginning 'aapyaayantu.'

31 upanishhads are from Atharva veda and have the Shaanti beginning bhadram-karNebhiH.

10 upanishhads are from Rig veda and have the Shaanti beginning vaNme-manasi.

The list of the 108 Upanishads can be found in Muktika 1: 30-39. Please note that the classification of each Upanishad is not give in the muktika.