The Katha Upanishad is not one of the four main Upanishads. The four main Upanishads are the Brihadaranyaka Upanishad, the Chandogya Upanishad, the Taittiriya Upanishad, and the Aitareya Upanishad.

The Upanishads teach that the self is the ultimate reality, that the world is an illusion, and that the universe is governed by a single, unified principle. These teachings are often referred to as the three pillars of the Upanishads.

Brahman is the term used in the Upanishads to refer to the ultimate reality. Brahman is described as being infinite, eternal, and unchanging. It is the source of all existence and the goal of all spiritual seeking.

Atman is the term used in the Upanishads to refer to the individual self. Atman is described as being immortal, unchanging, and blissful. It is the true nature of the self and the goal of all spiritual seeking.

Maya is the term used in the Upanishads to refer to the illusion of the world. Maya is described as being a veil of ignorance that prevents us from seeing the true nature of reality. It is the cause of all suffering and the obstacle to spiritual liberation.

Samsara is the term used in the Upanishads to refer to the cycle of birth, death, and rebirth. Samsara is described as being a wheel of suffering that we are trapped in until we achieve spiritual liberation.

The goal of spiritual seeking in the Upanishads is to attain moksha, which is liberation from the cycle of samsara and union with Brahman. This is achieved through the practice of yoga and meditation.

The Chandogya Upanishad contains the famous teaching, "Tat tvam asi" (Thou art That).

The Taittiriya Upanishad contains the teaching of the five sheaths of the self.

The Mandukya Upanishad contains the teaching of the four states of consciousness.

The Svetasvatara Upanishad contains the teaching of the three gunas.

The Kena Upanishad contains the teaching of the seven chakras.

The Kundalini Upanishad contains the teaching of the kundalini energy.

The Yoga Upanishad contains the teaching of the nadis.

The Prana Upanishad contains the teaching of the prana.