The Ishopanishad belongs to Shukla Yajurveda. It has only 18 verses. This small scripture embodies the best of Indian philosophy. Gandhiji once said, “…if only the first verse in the Ishopanishad were left in the memory of the Hindus, Hinduism would live forever.”
The 2nd verse of Isha Upanishad says,
“Doing verily works in this world one should wish to live a hundred years. Thus it is in thee and not otherwise than this; action cleaves not to a man.”
The Gita in chapter 2, Verse 47 says,
“You have the right to work only, and not to the fruits of work. Let not the fruit of action be your motive, nor let your attachment be to inaction.”

Source: Isha Upanishad translation: Sri Aurobindo; Gita translation: Vivekananda; Gandhi quote: Eknath Easwaran

Kena Upanishad triggers inquisitiveness with its verses posing insightful questions.
“By whom willed and directed does the mind light on its subjects?
By whom commanded does prana, the first, move?
By whose will do men speak this speech?
What Intelligence directs the eye and the ear?”
Kena is part of Samaveda and has 34 verses in 4 khandas. It shows how it is difficult for intellect to grasp the nature of supreme reality. In verse 1 of khanda 2 it says,
“The teacher said: If you think: “I know Brahman well,” then surely you know but little of Its form; you know only Its form as conditioned by man or by the gods. Therefore Brahman, even now, is worthy of your inquiry.”
Source: https://shlokam.org/texts/

Katha is part of Krishna Yajurveda and has 120 verses in 6 vallis (sections). It contains the famous dialogue between the boy Nachiketa and Yama, the God of Death.
Ratha Kalpana comes from Sanskrit ratha ‘chariot’ and kalpana ‘image.’ It is used in Hindu scriptures to describe the relationship between the senses, mind, intellect and the Self. The metaphor was first used in the Katha Upanishad. It may have inspired similar descriptions in the Bhagavad Gita, the Buddhist Dhammapada and the Greek work Phaedrus by Plato.
Katha Upanishad says,
“Know the atman as the lord of the chariot, the body as only the chariot, know also intelligence as the driver; know the minds as the reins.”
The narrative underscores the importance of self-knowledge. It also stresses that exceptional qualities are required to attain this knowledge, like Nachiketa had.

Source: ‘Upanishad Mala’, Chinmaya International Foundation

Prasna Upanishad, attached to the Atharva Veda, is the dialogue between Rishi Pippalada and his six disciples. It is in 6 sections with 67 mantras in all. Each section explores a facet of the ultimate reality. The 5th section details Omkara upasana.
“He replied: O Satyakama, the syllable AUM is the Supreme Brahman and also the other Brahman. Therefore he who knows it attains, with its support, the one or the other.”
The other or lower Brahman is said to be Prana or vital life force. The Supreme Brahman is beyond contemplation and thus meditation on Om is the best way to comprehend Brahman & Prana.
Om continues to be an important symbol in other Indic religions like Jainism & Buddhism. The most popular mantra in Tibetan Buddhism is “Om Mani Padme Hum…”, which is considered a condensed form of all Buddhist teaching.
Source: ‘Upanishad Mala’, Chinmaya International Foundation

Manduka Upanishad says, “Truth Alone Triumphs; not falsehood. Through truth the divine path is spread out by which the sages whose desires have been completely fulfilled, reach to where is that supreme treasure of Truth.”
Mundaka, related to Atharvaveda, consists of 65 verses taught by Rishi Angiras. The name comes as the chapters are called Mundakas which literally means ‘shaved’ or ‘liberated.’ It perhaps means that the contents can liberate a person.
It begins by asking what that is by knowing which everything else can be known. It posits two kind of knowledge – para that liberates by realization of reality and apara which is knowledge of everything else. It describes the Universe arising from the Supreme just as a spider spins its web making Brahman both the efficient & material cause, an intriguing philosophical proposition. It also presents the famous two bird metaphor. Two birds are shown to dwell on the same tree as inseparable companions. One tastes the fruits in the tree while the other merely watches. The former represents the individual self (Jivatman) and the latter the Supreme soul (Parmatman).
Source: ‘Upanishad Mala’, Chinmaya International Foundation

Mandukya is the shortest Upanishad with just 12 mantras. It teaches that the word OM represents the universe. Everything is Brahman and so is the self – the mahavakya ‘Ayam Atma Brahma.’
The self can be represented by four states of existence which are related to the syllables of the divine word OM – ‘A’, ‘U’, ‘M’ and a fourth soundless syllable.
The waking state is Vaisvanara which is related to ‘A’, the dream state is Taijasa related to ‘U’, the deep sleep is Prajna related to ‘M’ and there is a fourth state Chaturtha (later called Turiya by Gaudapada in his famous commentary on Mandukya) which is indescribable.
Verse 7 describes turiya as
“That is known as the fourth quarter: neither inward-turned nor outward-turned consciousness, nor the two together; not an undifferentiated mass of consciousness; neither knowing, nor unknowing; invisible, ineffable, intangible, devoid of characteristics, inconceivable, indefinable, its sole essence being the consciousness of its own Self; the coming to rest of all relative existence; utterly quiet; peaceful; blissful: without a second: this is the Ātman, the Self; this is to be realised.”
Source: Swami Krishnananda’s translation.

Numerous personality theories exist. Human personality is a combination of physical/ mental and spiritual dimensions. The Taittiriya Upanishad describes human beings having a five sheath personality (Pancha Kosha). They are 1) material or gross body (Anamaya Kosha), 2) the vital or instinctual component (Pranamaya Kosha), 3) the mental or psychological component (Manomaya Kosha), 4) the intellectual component (Vigyanmaya Kosha), and 5) pure bliss and happiness (Anandmaya Kosha).
The five Koshas of man are described in order to destroy the veil of ignorance. Each kosha leads to the next, each effect from its immediate cause, till the ultimate cause is reached. Taittirya from the Krishna Yajurveda has 31 mantras and in the writing of Swami Chinmayananda, “… occupies an unrivalled place in the Divine Tradition of Hinduism. A manifesto upon the ‘Hindu way of Living’”

sivanandaonline.org; Dr Biswajit Satpathy, ‘Pancha Kosha Theory of Personality’, IIJP

This is one of the few Upanishads whose author is known to us, Rishi Maheedasa Aitareya. Not much is known about his life. Sayana in his 14th cent CE commentary on Vedas narrates an interesting anecdote. The Rishi’s mother was Itara, which means “the other one.” She was probably the ‘other’ wife of Rishi Vishaala. When he was 5 years old Maheedasa was not allowed to sit on his father’s lap during a ceremony. This disturbed the mother, a potter-woman, who then prayed to Mother Earth. The boy became a great Rishi, with the name ‘Maheedasa’ – “servant of Mother Earth.”

This Upanishad has special appeal to the common person. It advises ‘charaiveti’ – keep moving, a practical logic.
“A moving person finds honey, a moving person tastes delicious fruits. Look at the sun’s effort, tirelessly moving every day without a hint of laziness. Therefore, keep moving, keep moving.”
The Upanishad has great prestige as it is attached to the Rig Veda and has 33 mantras. The mahavakya, “Prajnanam Brahma” i.e. Consciousness is Brahman, is from Aitareya Upanishad.

Source: Swami Gurubhaktananda, ‘Aitareya Upanishad’, Sandeepany Sadhanalaya. The picture shows a Charaiveti T Shirt from Amazon, testimony to its popular appeal.

Chandogya Upanishad tells a poignant story. One day a young boy visited Rishi Haridrumata Gautama and wanted to be his student. The sage asked, “Dear boy, of what gotra or lineage are you?” The boy replied that he does not know. He narrated his mother’s words, ‘I also do not know of what gotra you are. I used to serve many people and I got you in my youth. So I am not sure of what lineage you are. However, I am Jabala by name and you are Satyakama.’ On hearing it, the Rishi smiled and said, “No one who is not a Brahmin can speak thus. I shall initiate you as a Brahmacharin, for you have not deviated from truth.”
Attached to Sama Veda, Chandogya is the second largest and an old Upanishad. It provides the mahavakya, “Tat Tvam Asi i.e. Thou Art That.”
It provides the most popular metaphors to practically explain the relationship of self to the Supreme – the relationship of clay to a pot and that of gold ingot to jewellery. It also has the first reference to Sri Krishna outside the Mahabharata.
Source: Upanishads.org.in; ‘Upanishad Mala’, Chinmaya International Foundation. The image is from https://thephilosophicalmultiverse.com/the-story-of-satyakama-jabala/

Brhadaranyaka or the Great Forest Teaching, is attached to the Sukla Yajurveda. It is the largest and oldest Upanishad. It contains a series of dialogues between the great Upanishadic sage Yajnavalkya and many interlocutors including the outstanding woman scholars – Gargi & Maitreyi. Two colleges in Delhi are named after these women.
As the oldest Upanishad, it has the beginning of many philosophical ideas expanded later, like the different states of consciousness, importance of OM etc. It has the popular prayer (shanti mantra) , “Asato Ma Sadgamaya ..” i.e. “Lead Us From the Unreal To the Real, Lead Us From Darkness To Light, Lead Us From Death To Immortality, Let There Be Peace Peace Peace.” It also has the Mahavakya “Aham Brahmasmi.”
But its outstanding philosophical contribution is the idea of ‘neti neti.’ Vivekananda said, “We sometimes indicate a thing by describing its surroundings. When we say “Sachchidananda” (Existence-Knowledge-Bliss), we are merely indicating the shores of an indescribable Beyond. Any imagination, any concept is in vain. Neti, neti (“Not this, not that”) is all that can be said, for even to think is to limit and so to lose.”
Source: Vivekavani.com. The image from neevselfinquiry.in shows Gargi Vachaknavi debating Rishi Yajnavalkya under the watchful eye of King Janaka.