In contrast to the earlier Vedas, which are largely mantras for rituals, the 200 or so Upaniṣads represent the more esoteric, intuitive side—inner knowledge passed on through ṛsis—of the exoteric sacrifice literature of the earlier Vedas.
Thus the Upaniṣads suggest a “secret transmission” of the inner knowledge of the Vedas, passed on through the ṛṣīs. Of the many Upaniṣads, most were added later.
The Upaniṣads contain the major philosophical principles known in Hinduism today. These principles are: ātman (self) and its relation to brahman (universe) as an impersonal version of God. These Upaniṣads also describe God as the personal (visible) form of brahman. These scriptures also allude to karma, and even early forms of yoga.