Upanishads and Vedas are two terms that are often confused as one and the same thing. Actually they are two different subjects for that matter. In fact Upanishads are parts of Vedas.
Rig, Yajur, Sama and Atharva are the four Vedas. A Veda is divided into four parts, namely, Samhita, Brahmana, Aranyaka and Upanishad. It can be seen from the division that Upanishad forms the last part of a given Veda. Since Upanishad forms the end part of a Veda it is also called as Vedanta. The word ‘anta’ in Sanskrit means ‘end’. Hence the word ‘Vedanta’ means ‘the end portion of a Veda’.
The subject matter or the content of the Upanishad is normally philosophical in nature. It speaks about the nature of the Atman, the greatness of the Brahman or the Supreme Soul and also about the life after death. Hence Upanishad is called as the Jnana Kanda of the Veda. Jnana means knowledge. Upanishad speaks about the supreme or the highest knowledge.
The other three parts of the Veda, namely, Samhita, Brahmana and the Aranyaka are called together as Karma Kanda. Karma in Sanskrit means ‘action’ or ‘rituals’. It can be understood that the three parts of the Veda deal with the ritualistic part of life such as the conduct of a sacrifice, austerity and the like.
Veda thus contains in it both the ritualistic and the philosophical aspects of life. It deals with the actions to be performed in life and also with the spiritual thoughts that man should cultivate in his mind to read God.
Upanishads are many in number but only 12 of them are considered principal Upanishads. It is interesting to note that Adi Sankara, the founder of Advaita system of philosophy has commented on all the 12 principal Upanishads. The other major teachers of various sects of philosophical thoughts have quoted a lot from the texts of the Upanishads.