The Upanishads ("Sittings Near [a Teacher]") is part of the Hindu Shruti; these religious scriptures primarily discuss philosophy and "cosmic reality"; they also contain transcripts of various debates. There are 123 books argued to be part of the Upanishads; however, only 13 are accepted by all Hindus. Prominent topics, which are discussed within these texts, are those regarding Atman and Brahman.
The term, Upanishad, is derived 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 123 [[book]s, which constitute the Upanishads, are sometimes argued to be a part of the Vedas; and are thus known as Vedenta ("End of the Veda").
The oldest and longest of the Upanishads are the Brihad-Aranyaka and the Chandogya from about the 7th century B.C. Initially there were over two hundred upanishads but only fifteen were considered important by the philosopher Shankara.
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 upanishhads can be found in Muktika 1: 30-39. Please note that the classification of each upanishhad is not give in the muktika.