(1) Upanishad means "sitting near to teacher to receive instruction"; Up=near, Ni=down, and Shad=sit. They were so called because they were taught to those people who sat down near their Guru [to take them to near God].

(2) A collection of works of human thoughts of all times in respect to God, man, the Universe, the individual soul, and the relationship between them. They deal with the nature and purpose of existence, the ultimate reality and the attainment of beatitude. This literature arose to impart knowledge to those who wanted to know philosophical knowledge. Upanishad establish the identity of individual soul with the Supreme Soul and consider the human body as the temple housing God (the Supreme Soul) in the form of the individual soul. They say that God is bigger than the biggest, and subtler than the subtlest. He is unborn, eternal and without attributes. He cannot be defined or known. They are written partly in prose and partly in verse. The early Upanishad are concerned with sacrificial rites.

Upanishad are one of the five scriptural texts chosen by the ancients for constant repetition and meditation. These five texts are the Upanishads, The Bhagavad Geetaa, The Vishnu Sahasranaam, Shree Rudram and Purush Sooktam.

There are over a 100 so-called Upanishad in existence, some having been written as late as the 18th century AD. This is so because anyone could write a book on the subject and call it Upanishad. In truth, there are only 14 genuine Upanishad which can claim serious attention based upon the quality of their contents. These all were written during 1700-1100 BC (Aangiras' "The Upanishads", vol II, says that they were written during the 1100-700 BC) and are called classical Upanishad. These classical Upanishad are - 

(1) Aitareeya. (2) Brihadaaranyak. (3) Chhandogya. (4) Eeshaavaasya (Eesh). (5) Kath. (6) Kaivalya.
(7) Kausheetkee. (8) Ken. (9) Maitree or Maitraayanee. (10) Maandookya. (11) Mundak. (12) Prashn.
(13) Shwetaashwatar. (14) Taittireeya.

Among them 

There are 108 Upanishads, listed below, as per the list contained in the Muktikopanishad. They are arranged them in four categories according to the particular Ved to which each of them belong.

Atharv Ved (32):  (1) Prashn,  (2) Maandookya,  (3) Mundak,  (4) Aatm,  (5) Soorya,  (6) Naarad-Parivraajak,  (7) Parabrahm,  (8) Paramahans-Parivraajak,  (9) Pashupat-Brahma,  (10) Mahaavaakya,  (11) Shaandilya,  (12) Krishn,  (13) Garud,  (14) Gopaal-tapanee,  (15) Tripad-vibhuti-mahaanaaraayan,  (16) Dattaatreya,  (17) Kaivalya,  (18) Narasinh-tapanee,  (19) Raam-tapanee,  (20) Raam Rahasya,  (21) Hayagreev,  (22) Atharv-shikhaa,  (23) Atharv-shiraa,  (24) Ganapati, (25) Brahajjabal,  (26) Bhasm-Jaabaal,  (27) Sharabh,  (28) Annapoornaa, (29) Tripur-tapanee,  (30) Devee,  (31) Bhaavanaa,  (32) Seetaa.

Geetaa As An Upanishad
Although Bhagavad Geetaa is not an Upanishad, but in fact, it is the most important Upanishad. The Mundak Upanishad contains a verse which is the germ of the Bhagavad-Gita - "People who perform actions and are attached to the world are pursuing a futile path."; and This Upanishad, accordingly, declares: "Let the wise man, having examined the world and perceived the motives and the results of actions, realize that as from a blazing fire sparks proceed, living souls originate from the indestructible Brahm and return to Him. All doubts disappear and the attachment to work subsides when the Supreme Being is cognized."

Among the better known minor Upanishad are (1) Jaabaal, (2) Subaal, (3) Paingal, (4) Praanaagnihotra, (5) Naaraayan. The people, who call their works as Upanishad, are because the Prince Daaraa Shikoh, the eldest son of the Emperor Shaah Jahaan, who was an ardent student of Upanishad and had translated them into Persian language, ordered a new Upanishad composed and named it "Allopanishad" in praise of "Allaah" - the God of Muslim people.

The teachings of the Upanishad are the source of the entire Hindi thought and devotion. Saankhya, Yog, Meemaansaa and all three variants of Vedaant have drawn inspiration from the Upanishad. The Upanishad are diverse in character and outlook. They recognize intuition rather than reason as a path to ultimate truth. According to Kath Upanishad - "The knowledge of the Supreme is not gained by argument but by the teaching of one who possesses intuition."

They also represent a strong reaction against the merely ritual and sacrificial duties which were stressed earlier. Upanishad begins with the statement that whatever exists in this world is enveloped by the Supreme. It is by renunciation and absence of possessiveness that one can get Salvation. It is interesting to know that, according to the Chhaandogya Upanishad which is one of the earliest Upanishad, the main doctrines of the Upanishad were first expounded by the Kshatriya and not by the Brahmsn. Later, as is evident from the Kaushitakee Upanishad, the Braahman took up the intensive study of philosophy. The contrast which is often drawn between Braahmanism and Hinduism is therefore not based on a right appraisal of the facts.

Aadi Shankaraachaarya (8th century AD) preached that all phenomenal existence was illusion arising from A-Vidyaa (ignorance). While he borrowed the word Maayaa from Shwetaashwatar Upanishad, where it is used only twice, he assigned his own meaning to the word, translating it as "illusion" (that which is apparent but is unreal). Although he deviated from Upanishad, but he claimed that his doctrine was based on Upanishad.