The Atharvaveda is not one of the four Vedas. It is a collection of hymns and spells that is considered to be a later addition to the Vedic canon.

The Upanishads teach that the soul is immortal, the world is an illusion, and God is the creator of the universe. These teachings are considered to be the foundation of Hindu philosophy and religion.

The Brihadaranyaka Upanishad is considered to be the oldest Upanishad. It is a collection of philosophical dialogues that explore the nature of reality, the soul, and the universe.

The Brihadaranyaka Upanishad is known for its teaching of the concept of Brahman, which is the ultimate reality and the source of all existence.

The Chandogya Upanishad is known for its teaching of the concept of Atman, which is the individual soul or self.

The Kena Upanishad is known for its teaching of the concept of Maya, which is the illusion of the world.

The Katha Upanishad is known for its teaching of the concept of Karma, which is the law of cause and effect.

The Brihadaranyaka Upanishad is known for its teaching of the concept of Moksha, which is liberation from the cycle of birth and death.

The Chandogya Upanishad is known for its teaching of the concept of Jnana, which is knowledge of the ultimate reality.

The Katha Upanishad is known for its teaching of the concept of Bhakti, which is devotion to God.

The Brihadaranyaka Upanishad is known for its teaching of the concept of Yoga, which is a spiritual practice that aims to unite the individual soul with the ultimate reality.

The Brihadaranyaka Upanishad is known for its teaching of the concept of Vedanta, which is a school of Hindu philosophy that emphasizes the importance of knowledge and self-realization.

The Brihadaranyaka Upanishad is known for its teaching of the concept of Advaita Vedanta, which is a school of Hindu philosophy that emphasizes the non-duality of the individual soul and the ultimate reality.

The Chandogya Upanishad is known for its teaching of the concept of Dvaita Vedanta, which is a school of Hindu philosophy that emphasizes the duality of the individual soul and the ultimate reality.

The Brihadaranyaka Upanishad is known for its teaching of the concept of Vishishtadvaita Vedanta, which is a school of Hindu philosophy that emphasizes the qualified non-duality of the individual soul and the ultimate reality.