The Atharvaveda is not considered to be one of the four Vedas, but rather a collection of hymns and spells used for various purposes.

The Upanishads teach that the soul is immortal, the world is an illusion, and God is the creator of the universe.

Brahman is the term used in the Upanishads to refer to the Absolute Reality, the ultimate source and ground of all existence.

The Upanishads teach that Brahman and Atman are both aspects of the same reality, with Brahman being the universal aspect and Atman being the individual aspect.

The Upanishads teach that the goal of spiritual practice is to achieve liberation from the cycle of birth and death, to attain union with Brahman, and to become enlightened.

The Brihadaranyaka Upanishad is considered to be the most important and influential Upanishad due to its comprehensive and profound teachings on the nature of reality, the self, and the universe.

Maya is the term used in the Upanishads to refer to the world as an illusion, a veil of ignorance that obscures the true nature of reality.

The Chandogya Upanishad contains the famous teaching 'Tat Tvam Asi', which means 'Thou art That' and expresses the unity of the individual soul with the Absolute Reality.

Karma is the term used in the Upanishads to refer to the law of cause and effect, which determines the individual's destiny and is the means to achieve liberation.

The Katha Upanishad is known for its emphasis on the importance of self-knowledge, as expressed in the famous teaching 'Know thyself'.

Moksha is the term used in the Upanishads to refer to liberation, enlightenment, and union with Brahman, which is the ultimate goal of spiritual practice.

The Isa Upanishad is known for its concise and poetic teachings, which are often expressed in the form of aphorisms.

Renunciation is the term used in the Upanishads to refer to the giving up of material possessions, the detachment from worldly desires, and the practice of self-denial as a means to achieve spiritual liberation.

The Mandukya Upanishad is known for its emphasis on the importance of meditation, particularly the practice of 'Om' meditation.

Avidya is the term used in the Upanishads to refer to the ignorance of the true nature of reality, which is the cause of suffering and the obstacle to liberation.